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DB represents a handle to database at application level and contains pool
of connections. DB.Open opens database connection. The connection will be
automatically put back into DB pool for future reuse after corresponding
transaction is complete. DB thus provides service to maintain live objects
cache and reuse live objects from transaction to transaction.

Note that it is possible to have several DB handles to the same database.
This might be useful if application accesses distinctly different sets of
objects in different transactions and knows beforehand which set it will be
next time. Then, to avoid huge cache misses, it makes sense to keep DB
handles opened for every possible case of application access.

TODO handle invalidations.