When a DB gets deleted, we clear out its metadata. This included sysmeta
such as that used to tell shards the name of their root DB.
Previously, this would cause deleted shards to pop back to life as roots
that claimed to have objects still sitting in whatever container they
sharnk into.
Now, use the metadata if it's available, but when it's not, go by the
state of the DB's "own shard range" -- deleted shards should be marked
deleted, while roots never are.
This allows us to actually clean up the database files; you can test
this by doing something like
* Run `nosetests test/probe/test_sharder.py:TestContainerSharding.test_shrinking`
* Run `find /srv/*/*/containers -name '*.db'` to see how many databases
are left on disk. There should be 15: 3 for the root container, 6 for
the two shards on the first pass, and another 6 for the two shards on
the second pass.
* Edit container configs to decrease reclaim_age -- even 1 should be
fine.
* Run `swift-init main start` to restart the servers.
* Run `swift-init container-sharder once` to have the shards get marked
deleted.
* Run `swift-init container-updater once` to ensure all containers have
reported.
* Run `swift-init container-replicator once` to clean up the
containers.
* Run `find /srv/*/*/containers -name '*.db'` again to verify no
containers remain on disk.
Change-Id: Icba98f1c9e17e8ade3f0e1b9a23360cf5ab8c86b
a4cf508a38