Add DatabaseSet wrapper class and use it.

Introduce a new class DatabaseSet that wraps around the "blockchain
status databases" CTxDB and CNameDB.  Use it instead of the classes
themselves for blockchain validation.  The only net effect this has is
that CNameDB is not created/destroyed in each Connect/DisconnectInputs
hook, but at the same time as the CTxDB.  It possibly also fixes syncing
between both with atomic transactions.