Improve version comparison functions, esp for PostgreSQL

[Russell Keith-Magee]

The PostgreSQL backend currently implements a get_version function that returns a tuple (e.g., (8,3,1) ) that describes the version of the PostgreSQL server. This version tuple is then used in many locations to determine feature availability - for example, between 8.2 and 8.2.4, STDDEV_POP is known to be broken, and errors will be raised.

Version comparisons are implemented in multiple locations, using different techniques. This should be cleaned up and improved so that the process of identifying versions can be improved without affecting the version comparisons that are required. This probably means writing a function that returns -1/0/1 comparing one version to another version.

This is an offshoot of #10842 - more discussion and context is available on that ticket.

[Ramiro Morales]

In [16439]:

Fixed #11065, #11285 -- Streamlined PostgreSQL version detection, fixing incompatibility with multi-db. Thanks findepi for the report.

Changed our internal representation of the PostgreSQL version from tuples to
integers as used by libpq and psycopg2. This simplifies version comparison
operations.

Also, using the associated libpq/psycopg2 API allows to remove the need for
manually issuing in-band SELECT version() SQL queries to obtain the server
version (or at least reduce its number if version of psycopg2 in use is older
than 2.0.12). Refs #10509.