Specifying SourceShard better, easier.
Merge branch 'master' into resharding
Restoring the uid as a unique id for resharding.

It is used as a unique id for a SourceShard, as well
as the primary key on _vt.blp_recovery.
Also checking schema in resharding.py.
Merge commit 'f8cd848c430e72f533a2b614214391e147c92a2f'