Handle PostgreSQL transaction errors and improve schema detection

Handle PostgreSQL transaction errors and improve schema detection

* Introduce $wgDebugDBTransactions facility to help
  figure out what's going on with transactions.
  Currently PostgreSQL only.

  PostgresTransactionState can be easily be made more general
  to trace all sorts of state machinery.

* Improve r113408: we don't need to full reconnect on error,
  rollback is enough.

  Rolling back breaks search_path, as PostgreSQL
  can manage sessions settings under transaction therefore
  we need to improve schema sniffing introduced in r82674

* Introduce few schema handling functions. This could
  probably be generalized for other databases like DB2 and Oracle.

* Fix bug 15816 - Add a switch for SETting the search_path

  We try to avoid touching search_path at all unless
  really necessary. Even in this case we append MediaWiki
  core schema to the front of the list.

* No longer add $wgDBmwschema to PostgreSQL role search_path
  in the installer. This is no longer necessary as
  setting schema on connect should ReallyWorkNow(tm).

* Get rid as much as possible of $wgDBmwschema and
  bring us one step closer to fix bug 16794 (wgSharedDB
  support).

  All references to core MediaWiki schema in PostgreSQL
  specific code should now use Database::getCoreSchema()
  unless we know what we are doing.

Followup-To: r113408 r82674