Thread safe handling of CORAL transactions by RDBAccessSvc

Given CORAL transaction context seems to be global, one thread can commit a transaction, which is active at that moment in another thread. Worked around this issue by starting read-only transaction when a session is created, and committing that transaction only when the corresponding session is closed.

This addresses the issue discussed in ATLASRECTS-5737.

Other changes:

• Removed several redundant include statements;
• Applied thread-safety fixes to the way RDBQuery handles database connection;
• Implemented a minor reorganization of the internal cache of RDBAccessSvc