rdbms: normalize Database open() code and error handling

rdbms: normalize Database open() code and error handling

Mainly:
* Use oci_new_connect() for Oracle to avoid broken connection reuse
  similar to the PGSQL_CONNECT_FORCE_NEW flag in DatabasePostgres
* Set 'client_min_messages' unconditionally for PostgreSQL
* Factor out Database::getConnectExceptionAndLog() helper method
* Use the same style of query() calls in DatabaseOracle::open() as
  the other subclasses
* Make sure the Database driver handle field is null on failure
  instead of false for sanity

Also:
* Disallow changing of Database handle DBO_* flags after construction
  where it does not make sense to change them
* Do not mention DBO_* flags meant for non-config use in $wgDBservers
* Ignore DBO_PERSISTENT for SQLite if DBO_TRX is also set for sanity
* Remove $wgDBOracleDRCP variable to discourage careless automatic
  setting of DBO_PERSISTENT that breaks LoadBalancer assumptions

Change-Id: Iea948f7f872294ea8fc5d897fc10c9d29b7141d5