TokyoCabinet pseudo-environment

TokyoCabinet pseudo-environment

Since TokyoCabinet does not provide a DB environment, trying to reopen
the same db multiple times (something that happens with subregistry
mostly, and possibly after rescans) causes the subsequent opens to fail
due to "threading errors".

Fix by implementing some sort of environmentalish database management
(only keeps track of open databases, returns existing ones if reopening
the same db multiple times, closes all of them on exit).