Drop the GIL while running sync

[djmitche]

A sync may take a while, but doesn't touch anything accessible from Python. We could drop the GIL while syncing!

Since Replica is currently not Send (#25), this would be safe: no other thread can call a method on a Replica while the sync is ongoing, since it is using the only thread allowed to do so.

However, if we ever drop that restriction then we'd need to add some other kind of mutual exclusion to ensure the Replica isn't modified while sync is running. So, I'm not sure it's a good idea to drop the GIL just yet.

If someone has a case for doing so, please comment here.