Throw if WAL enabled/disabled when connections are in use.

Changing WAL mode requires obtaining an exclusive lock on the
database and can only be done when there are NO other active
database connections.

Check that this is really the case, and bail with a useful
error message if an application attempts to change WAL mode while
transactions are in progress.

Expose disableWriteAheadLogging() in the API.

Change-Id: I87599de3b88c53dcd75677aefd72e40de216c2c1