* rpc: simplify the handling of JSON-RPC request and response IDs

Replace the ID wrapper interface with plain JSON. Internally, the client
libraries use only integer IDs, and the server does not care about the ID
structure apart from checking its validity.

Basic structure of this change:

- Remove the jsonrpcid interface and its helpers.
- Unexport the ID field of request and response.
- Add helpers for constructing requests and responses.
- Fix up usage and tests.

Responses are constructed from requests using MakeResponse, MakeError, and
MakeErrorf. This ensures the response is always paired with the correct ID,
makes cases where there is no ID more explicit at the usage site, and
consolidates the handling of error introspection across transports.

The logic for unpacking errors and assigning JSON-RPC response types was
previously duplicated in three places. Consolidate it in the types package for
the RPC subsystem.

* update test cases

Instead of having the exported Request and Response type expose the version
directly, delegate version injection to an unexported shim.  We already had the
shim; this just boosts it to the top level and does a bit more checking.

* Check JSON-RPC version marker.

The main change here is to use encoding/json to encode and decode RPC 
parameters, rather than the custom tmjson package. This includes:

- Update the HTTP POST handler parameter handling.
- Add field tags to 64-bit integer types to get string encoding (to match amino/tmjson).
- Add marshalers to struct types that mention interfaces.
- Inject wrappers to decode interface arguments in RPC handlers.

* Rename rpctypes.Context to CallInfo.

Add methods to attach and recover this value from a context.Context.

* Rework RPC method handlers to accept "real" contexts.

- Replace *rpctypes.Context arguments with context.Context.
- Update usage of RPC context fields to use CallInfo.

* Update Caller interface and its documentation.
* Rework MapToRequest as ParamsToRequest.

The old interface returned the result as well as populating it.  Nothing was
using this, so drop the duplicated value from the return signature. Clarify the
documentation on the Caller type.

Rework the MapToRequest helper to take an arbitrary value instead of only a
map. This is groundwork for getting rid of the custom marshaling code. For now,
however, the implementation preserves the existing behaviour for the map, until
we can replace those.

Apart from the tests for the websocket client, positional parameters are not
used by RPC clients. The server supports both arrays and objects, but the
client only needs to provide one or the other.

This continues the push of plumbing contexts through tendermint. I
attempted to find all goroutines in the production code (non-test) and
made sure that these threads would exit when their contexts were
canceled, and I believe this PR does that.

i.e. without double pointer. With double pointer, it was possible to
submit `null` value, which will crash the server.

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x189ddc0]

goroutine 1 [running]:
github.com/tendermint/tendermint/rpc/jsonrpc/types.(*RPCRequest).UnmarshalJSON(0xc0000147e0, 0xc00029f201, 0x4, 0x1ff, 0x883baa0, 0xc0000147e0)
        /Users/anton/go/src/github.com/tendermint/tendermint/rpc/jsonrpc/types/types.go:70 +0x100
encoding/json.(*decodeState).literalStore(0xc000216bb0, 0xc00029f201, 0x4, 0x1ff, 0x1998800, 0xc0000147e0, 0x199, 0xc000231700, 0x10e0a5e, 0x197)
        /usr/local/Cellar/go/1.16/libexec/src/encoding/json/decode.go:860 +0x30ce
encoding/json.(*decodeState).value(0xc000216bb0, 0x1998800, 0xc0000147e0, 0x199, 0x1998800, 0xc0000147e0)
        /usr/local/Cellar/go/1.16/libexec/src/encoding/json/decode.go:384 +0x40c
encoding/json.(*decodeState).array(0xc000216bb0, 0x18df040, 0xc0001be540, 0x16, 0xc000216bd8, 0x10e405b)
        /usr/local/Cellar/go/1.16/libexec/src/encoding/json/decode.go:558 +0x365
encoding/json.(*decodeState).value(0xc000216bb0, 0x18df040, 0xc0001be540, 0x16, 0x16, 0x6e)
        /usr/local/Cellar/go/1.16/libexec/src/encoding/json/decode.go:360 +0x22f
encoding/json.(*decodeState).unmarshal(0xc000216bb0, 0x18df040, 0xc0001be540, 0xc000216bd8, 0x0)
        /usr/local/Cellar/go/1.16/libexec/src/encoding/json/decode.go:180 +0x2c9
encoding/json.Unmarshal(0xc00029f200, 0x6, 0x200, 0x18df040, 0xc0001be540, 0x0, 0x0)
        /usr/local/Cellar/go/1.16/libexec/src/encoding/json/decode.go:107 +0x15d
```

* docs: goleveldb is much more stable now

Refs https://github.com/syndtr/goleveldb/issues/226#issuecomment-682495490

* rpc/core/events: make sure WS client receives every event

previously, if the write buffer was full, the response would've been
lost without any trace (log msg, etc.)

* rpc/jsonrpc/server: set defaultWSWriteChanCapacity to 1

Refs #3905
Closes #3829

setting write buffer capacity to 1 makes transactions count per block
more stable and also reduces the pauses length by 20s.

before: https://github.com/tendermint/tendermint/issues/3905#issuecomment-681854328 net.Read - 20s
after: net.Read - 0.66s

* rpc/jsonrpc/server: buffer writes and avoid io.ReadAll during read

Migrates the `rpc` package to use new JSON encoder in #4955. Branched off of that PR.

Tests pass, but I haven't done any manual testing beyond that. This should be handled as part of broader 0.34 testing.

Closes https://github.com/tendermint/tendermint/issues/3857

Moves `lib/` folder to `jsonrpc/`.

Renames:

**packages**

`rpc` package -> `jsonrpc` package
`rpcclient` package -> `client` package
`rpcserver` package -> `server` package

**structs and interfaces**

```
JSONRPCClient to Client
JSONRPCRequestBatch to RequestBatch
JSONRPCCaller to Caller
```

**functions**

```
StartHTTPServer to Serve
StartHTTPAndTLSServer to ServeTLS

rpc/jsonrpc/client: rename NewURIClient to NewURI

NewJSONRPCClient to New
NewJSONRPCClientWithHTTPClient to NewWithHTTPClient
NewWSClient to NewWS
```

**misc**

- unexpose `ResponseWriterWrapper`
- remove unused http_params.go