Sockets-touching bits copied and adapted from `CConnman::SocketSendData()`

Testing this requires adding a new feature to the SocketTestingSetup,
returning the DynSock I/O pipes from the mock socket so the received
data can be checked.

Co-authored-by: Vasil Dimov <vd@FreeBSD.org>

See https://www.rfc-editor.org/rfc/rfc7230#section-6.3.2

> A server MAY process a sequence of pipelined requests in
  parallel if they all have safe methods (Section 4.2.1 of [RFC7231]),
  but it MUST send the corresponding responses in the same order that
  the requests were received.

We choose NOT to process requests in parallel. They are executed in
the order recevied as well as responded to in the order received.
This prevents race conditions where old state may get sent in response
to requests that are very quick to process but were requested later on
in the queue.

This is a refactor to prepare for matching the API of HTTPRequest
definitions in both namespaces http_bitcoin and http_libevent. In
particular, to provide a consistent return type for GetRequestMethod()
in both classes.

These methods are called by http_request_cb() and are present in the
original http_libevent::HTTPRequest.

The original function is passed to libevent as a callback when HTTP
requests are received and processed. It wrapped the libevent request
object in a http_libevent::HTTPRequest and then handed that off to
bitcoin for basic checks and finally dispatch to worker threads.

In this commit we split the function after the
http_libevent::HTTPRequest is created, and pass that object to a new
function that maintains the logic of checking and dispatching.

This will be the merge point for http_libevent and http_bitcoin,
where HTTPRequest objects from either namespace have the same
downstream lifecycle.

The original function was already naturally split into two chunks:
First, we parse and validate the users' RPC configuration for IPs and
ports. Next we bind libevent's http server to the appropriate
endpoints.

This commit splits these chunks into two separate functions, leaving
the argument parsing in the common space of the module and moving the
libevent-specific binding into the http_libevent namespace.

A future commit will implement http_bitcoin::HTTPBindAddresses to
bind the validate list of endpoints by the new HTTP server.

This reverts commit 8d8e2c0.