covers_multiple, source_names, source and reader_for replace the private
state seven modules were reading to work out how many databases they had.
The configured selection is kept intact, so entering a client twice derives
the same database rather than the last derivation.
One session field, whichever kind it is, with the covered databases derived
from it rather than kept beside it. `_single_session` returns the database a
write works on in place of ten guards that only asserted one existed, and the
single-database operations move to the session that owns them.
FederatedSession opens the databases a query covers and owns their
teardown; the client keeps the wrappers it hands out. A wrapper releases
what it built and never closes the database it borrowed.
SingleDatabaseSession owns the store, the repositories and the vacuum
machinery, so nothing above has to ask whether it has a store. The client
keeps every name callers already use.
