Delete the old export dbs on upgrade.
Testing this an exporting to a directory with both exporttree=yes and
importtree=yes, it refused to let an interrupted export proceed after
upgrade, with "unsafe to overwrite file". An import resolved the
problem.
It will be populated automatically by the next command that needs data
from it, the same way it gets populated in a fresh clone. That may be a
little expensive, but it's a one time cost, and no slower than in a
fresh clone.
The old db is cleaned up when a new incremental fsck is started.
The incremental fsck won't pick up where the old one left off, but I
consider this a minor enough thing that it can just be documented and
won't be a problem.
Renamed the database to .git/annex/keysdb;
the old .git/annex/keys gets deleted during the upgrade.
It is possible that an old git-annex process is running during the
upgrade. If so, it will be able to continue using the old keys db until the
upgrade is complete, and then will presumably fail in some ugly way. Or
perhaps the upgrade will be unable to delete the open files on some
systems, and so fail with an ugly error message.
It's also possible for multiple processes to be running the upgrade
concurrently. That should be fine; they will both write the same
information into the keys db.
Other databases still need to be upgraded.
