rdbms: avoid connections on more lazy DBConnRef methods

rdbms: avoid connections on more lazy DBConnRef methods

Also:
* Fixed LoadBalancer::getAnyOpenConnection for both
  DB_MASTER and DB_REPLICA, which are not real indexes.
* Lock down DBConnRef::close since it can only cause trouble.
* Relax DBConnRef restrictions on tablePrefix()/dbSchema()
  for the harmless "getter" mode case.
* Remove redundant DatabasePostgres::getServer definition.

Change-Id: Ia855d901cc3c28147e52284fdabb1645805d4466