os/bluestore: move cached items around on collection split

os/bluestore: move cached items around on collection split

We've been avoiding doing this for a while and it has finally caught up
with us: the SharedBlob may outlive the split due to deferred IO, and
a read on the child collection may load a competing Blob and SharedBlob
and read from the on-disk blocks that haven't been written yet.

Fix by preserving the one-SharedBlob-instance invariant by moving cache
items to the new Collection and cache shard like we should have from the
beginning.

Signed-off-by: Sage Weil <sage@redhat.com>