Fix various PostgreSQL failures

Fix various PostgreSQL failures

* Fix schema for image_comment_temp.
* Provide values in CommentStoreTest::provideInsertRoundTrip() for
  columns where the PG schema doesn't have a default value but the MySQL
  schema does.
* Call nextSequenceValue() from CommentStoreTest::testInsertRoundTrip().
* Correctly handle $options being the string 'FOR UPDATE' in
  DatabasePostgres::selectSQLText()
* Correctly handle the initial table in DatabasePostgres::selectSQLText() FOR
  UPDATE mangling.
* Correctly handle aliases in DatabasePostgres::selectSQLText() FOR
  UPDATE mangling.

Tests in PG are still going to be broken thanks to the fact that
nextSequenceValue() and insertId() can't be separated by another
nextSequenceValue()/insertId() pair. That should be taken care of by
T164898/T164900.

Change-Id: Ia770a003ca9170ab8bcc1436d8afe30700e00ada