Re: Add non-blocking version of PQcancel

I attached a new version of this patch. Which does three main things:
1. Change the PQrequestCancel implementation to use the regular
connection establishement code, to support all connection options
including encryption.
2. Add PQrequestCancelStart which is a thread-safe and non-blocking
version of this new PQrequestCancel implementation.
3. Add PQconnectComplete, which completes a connection started by
PQrequestCancelStart. This is useful if you want a thread-safe, but
blocking cancel (without having a need for signal safety).

This change un-deprecates PQrequestCancel, since now there's actually an
advantage to using it over PQcancel. It also includes user facing documentation
for all these functions.

As a API design change from the previous version, PQrequestCancelStart now
returns a regular PGconn for the cancel connection.

@Tom Lane regarding this:
> Even in the non-blocking case, callers should only deal with the original PGconn.

This would by definition result in non-threadsafe code (afaict). So I refrained from doing this.
The blocking version doesn't expose a PGconn at all, but the non-blocking one now returns a new PGconn.

There's two more changes that I at least want to do before considering this patch mergable:
1. Go over all the functions that can be called with a PGconn, but should not be
called with a cancellation PGconn and error out or exit early.
2. Copy over the SockAddr from the original connection and always connect to
the same socket. I believe with the current code the cancellation could end up
at the wrong server if there are multiple hosts listed in the connection string.

And there's a third item that I would like to do as a bonus:
3. Actually use the non-blocking API for the postgres_fdw code to implement a
timeout. Which would allow this comment can be removed:
/*
* Issue cancel request. Unfortunately, there's no good way to limit the
* amount of time that we might block inside PQgetCancel().
*/

So a next version of this patch can be expected somewhere later this week.
But any feedback on the current version would be appreciated. Because
these 3 changes won't change the overall design much.

Jelte