Make DBConnRef enforce the DB domain selected during its lifetime
and allow more nested and successive use of the same connection handle
via DBConnRef. This can avoid extra connections in some cases where
getConnection()/getConnectionRef() is used.
Also:
* Reduce the number of connection pools arrays from six to two
* Merge getLocalConnection()/getForeignConnection() into one method
* Expand various related code comments
Bug: T226595
Depends-On: If808cbab429d41e1f2289683533e4a781a4bdf5e
Change-Id: I540b08920997c57cad6445ddb09d8e663eaf4714
76be4dca81