rdbms: remove $opened field from Database for simplicity

rdbms: remove $opened field from Database for simplicity

This avoids having two similar fields that have to stay
in sync. Clean up the related error handling for connections.
If a connection handle is unusable, like when essential SET
queries fail, then destroy it.

Also:
* Avoid use of transactions in DatabasePostgres::determineCoreSchema.
* Make sure all subclasses log on connection failure.
* Add schema sanity checks to mysql/sqlite classes.
* Add IDatabase::QUERY_NO_RETRY flag to simplify reasoning about
  queries that already run on open() to begin with.
* Remove unused return value of Database::open.
* Remove deprecated Database::reportConnectionError method.

Change-Id: I97beba7ead1523085bda8784234d00c69ef1accc