odb: adopt logic to close object databases

odb: adopt logic to close object databases

The logic to close an object database is currently contained in the
packfile subsystem. That choice is somewhat relatable, as most of the
logic really is to close resources associated with the packfile store
itself. But we also end up handling object sources and commit graphs,
which certainly is not related to packfiles.

Move the function into the object database subsystem and rename it to
`odb_close()`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>