Add non-blocking version of PQcancel

The existing PQcancel API is using blocking IO. This makes PQcancel
impossible to use in an event loop based codebase, without blocking the
event loop until the call returns.

This patch adds a new cancellation API to libpq which is called
PQcancelConnectionStart. This API can be used to send cancellations in a
non-blocking fashion. To do this it internally uses regular PGconn
connection establishment. This has as a downside that
PQcancelConnectionStart cannot be safely called from a signal handler.

Luckily, this should be fine for most usages of this API. Since most
code that's using an event loop handles signals in that event loop as
well (as opposed to calling functions from the signal handler directly).

There are also a few advantages of this approach:
1. No need to add and maintain a second non-blocking connection
establishment codepath.
2. Cancel connections benefit automatically from any improvements made
to the normal connection establishment codepath. Examples of things
that it currently gets for free currently are TLS support and
keepalive settings.

This patch also includes a test for this new API (and also the already
existing cancellation APIs). The test can be easily run like this:

cd src/test/modules/libpq_pipeline
make && ./libpq_pipeline cancel

NOTE: I have not tested this with GSS for the moment. My expectation is
that using this new API with a GSS connection will result in a
CONNECTION_BAD status when calling PQcancelStatus. The reason for this
is that GSS reads will also need to communicate back that an EOF was
found, just like I've done for TLS reads and unencrypted reads. Since in
case of a cancel connection an EOF is actually expected, and should not
be treated as an error.