rdbms: Remove support for PostgreSQL < 9.2, and improve INSERT IGNORE for 9.5

rdbms: Remove support for PostgreSQL < 9.2, and improve INSERT IGNORE for 9.5

MediaWiki doesn't support PostgreSQL < 9.2, so drop the support for
older versions.

At the same time, since we're messing with the DatabasePostgres::insert()
code anyway, let's start using ON CONFLICT DO NOTHING for PG >= 9.5.

And since we're doing that, let's do the same for
DatabasePostgres::nativeInsertSelect().

Change-Id: I7bf13c3272917ebafeaff11eb116714a099afdf3