* 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