pgsql: Support connection load balancing in libpq

Support connection load balancing in libpq

This adds support for load balancing connections with libpq using a
connection parameter: load_balance_hosts=<string>. When setting the
param to random, hosts and addresses will be connected to in random
order. This then results in load balancing across these addresses and
hosts when multiple clients or frequent connection setups are used.

The randomization employed performs two levels of shuffling:

1. The given hosts are randomly shuffled, before resolving them
one-by-one.
2. Once a host its addresses get resolved, the returned addresses
are shuffled, before trying to connect to them one-by-one.

Author: Jelte Fennema <postgres(at)jeltef(dot)nl>
Reviewed-by: Aleksander Alekseev <aleksander(at)timescale(dot)com>
Reviewed-by: Michael Banck <mbanck(at)gmx(dot)net>
Reviewed-by: Andrey Borodin <amborodin86(at)gmail(dot)com>
Discussion: https://postgr.es/m/PR3PR83MB04768E2FF04818EEB2179949F7A69@PR3PR83MB0476.EURPRD83.prod.outlook.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/7f5b19817eaf38e70ad1153db4e644ee9456853e

Modified Files
--------------
.cirrus.yml | 16 ++-
doc/src/sgml/libpq.sgml | 61 ++++++++++
doc/src/sgml/regress.sgml | 13 ++-
src/interfaces/libpq/fe-connect.c | 103 +++++++++++++++++
src/interfaces/libpq/libpq-int.h | 17 ++-
src/interfaces/libpq/meson.build | 2 +
.../libpq/t/003_load_balance_host_list.pl | 81 ++++++++++++++
src/interfaces/libpq/t/004_load_balance_dns.pl | 124 +++++++++++++++++++++
src/test/perl/PostgreSQL/Test/Cluster.pm | 16 +++
src/tools/pgindent/typedefs.list | 1 +
10 files changed, 431 insertions(+), 3 deletions(-)