TokyoCabinet pseudo-environment

TokyoCabinet pseudo-environment

TokyoCabinet does not have an environment feature to automatically
manage databases, and in particular there is no simple way to close all
open registries on exit. While we do close main registries for all
plugins, this is not usually done for sub-registries. Rather than
shifting the chore of their management to each plugin, we set up a list
of all opened registries, updating it manually and releasing it on
shutdown.