pgsql: libpq: Add encrypted and non-blocking query cancellation routine

libpq: Add encrypted and non-blocking query cancellation routines

The existing PQcancel API uses blocking IO, which makes PQcancel
impossible to use in an event loop based codebase without blocking the
event loop until the call returns. It also doesn't encrypt the
connection over which the cancel request is sent, even when the original
connection required encryption.

This commit adds a PQcancelConn struct and assorted functions, which
provide a better mechanism of sending cancel requests; in particular all
the encryption used in the original connection are also used in the
cancel connection. The main entry points are:

- PQcancelCreate creates the PQcancelConn based on the original
connection (but does not establish an actual connection).
- PQcancelStart can be used to initiate non-blocking cancel requests,
using encryption if the original connection did so, which must be
pumped using
- PQcancelPoll.
- PQcancelReset puts a PQcancelConn back in state so that it can be
reused to send a new cancel request to the same connection.
- PQcancelBlocking is a simpler-to-use blocking API that still uses
encryption.

Additional functions are
- PQcancelStatus, mimicks PQstatus;
- PQcancelSocket, mimicks PQcancelSocket;
- PQcancelErrorMessage, mimicks PQerrorMessage;
- PQcancelFinish, mimicks PQfinish.

Author: Jelte Fennema-Nio <postgres(at)jeltef(dot)nl>
Reviewed-by: Denis Laxalde <denis(dot)laxalde(at)dalibo(dot)com>
Discussion: https://postgr.es/m/AM5PR83MB0178D3B31CA1B6EC4A8ECC42F7529@AM5PR83MB0178.EURPRD83.prod.outlook.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/61461a300c1cb5d53955ecd792ad0ce75a104736

Modified Files
--------------
doc/src/sgml/libpq.sgml | 507 +++++++++++++++++++++--
src/interfaces/libpq/exports.txt | 9 +
src/interfaces/libpq/fe-cancel.c | 296 ++++++++++++-
src/interfaces/libpq/fe-connect.c | 129 +++++-
src/interfaces/libpq/libpq-fe.h | 31 +-
src/interfaces/libpq/libpq-int.h | 5 +
src/test/modules/libpq_pipeline/libpq_pipeline.c | 121 ++++++
src/tools/pgindent/typedefs.list | 1 +
8 files changed, 1044 insertions(+), 55 deletions(-)