We should not set cache-control headers on RPC responses. HTTP caching
interacts poorly with resources that are expected to change frequently, or
whose rate of change is unpredictable.

More subtly, all calls to the POST endpoint use the same URL, which means a
cacheable response from one call may actually "hide" an uncacheable response
from a subsequent one. This is less of a problem for the GET endpoints, but
that means the behaviour of RPCs varies depending on which HTTP method your
client happens to use. Websocket requests were already marked statically
uncacheable, adding yet a third combination.

To address this:

- Stop setting cache-control headers.
- Update the tests that were checking for those headers.
- Remove the flags to request cache-control.

Apart from affecting the HTTP response headers, this change does not modify the
behaviour of any of the RPC methods.

* 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.

No API changes, merely changes the import path.

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.

As a safety measure, don't allow a query string to be unreasonably
long. The query filter is not especially efficient, so a query that
needs more than basic detail should filter coarsely in the subscriber
and refine on the client side.

This affects Subscribe and TxSearch queries.

This is part of the work described by #7156.

Remove "unbuffered subscriptions" from the pubsub service.
Replace them with a dedicated blocking "observer" mechanism.
Use the observer mechanism for indexing.

Add a SubscribeWithArgs method and deprecate the old Subscribe
method. Remove SubscribeUnbuffered entirely (breaking).

Rework the Subscription interface to eliminate exposed channels.
Subscriptions now use a context to manage lifecycle notifications.

Internalize the eventbus package.

Addresses one of the concerns with #7041.

Provides a mechanism (via the RPC interface) to delete a single transaction, described by its hash, from the mempool. The method returns an error if the transaction cannot be found. Once the transaction is removed it remains in the cache and cannot be resubmitted until the cache is cleared or it expires from the cache.

A few notes:

- this is not all the deletion that we can do, but this is the most
  "simple" case: it leaves in shims, and there's some trivial
  additional cleanup to the transport that can happen but that
  requires writing more code, and I wanted this to be easy to review
  above all else.
  
- This should land *after* we cut the branch for 0.35, but I'm
  anticipating that to happen soon, and I wanted to run this through
  CI.

The code in the Tendermint repository makes heavy use of import aliasing.
This is made necessary by our extensive reuse of common base package names, and
by repetition of similar names across different subdirectories.

Unfortunately we have not been very consistent about which packages we alias in
various circumstances, and the aliases we use vary. In the spirit of the advice
in the style guide and https://github.com/golang/go/wiki/CodeReviewComments#imports,
his change makes an effort to clean up and normalize import aliasing.

This change makes no API or behavioral changes. It is a pure cleanup intended
o help make the code more readable to developers (including myself) trying to
understand what is being imported where.

Only unexported names have been modified, and the changes were generated and
applied mechanically with gofmt -r and comby, respecting the lexical and
syntactic rules of Go.  Even so, I did not fix every inconsistency. Where the
changes would be too disruptive, I left it alone.

The principles I followed in this cleanup are:

- Remove aliases that restate the package name.
- Remove aliases where the base package name is unambiguous.
- Move overly-terse abbreviations from the import to the usage site.
- Fix lexical issues (remove underscores, remove capitalization).
- Fix import groupings to more closely match the style guide.
- Group blank (side-effecting) imports and ensure they are commented.
- Add aliases to multiple imports with the same base package name.