Cleans up and organizes several scattered functions and variables related to
the BDB env. Class CDBInit() existed to provide a
guaranteed-via-C++-destructor cleanup of the db environment.
A formal CDBEnv class provides all of this inside a single wrapper.
This commit removes the dependency of serialize.h on PROTOCOL_VERSION,
and makes this parameter required instead of implicit. This is much saner,
as it makes the places where changing a version number can have an
influence obvious.
