Decide which, if any, steps in a sync are interruptible

[humphd]

In #304 we are adding code that allows a client to release a lock early if it receives a request from another client that wants to sync. The way to control this is to set lock.allowLockRequest=true in the areas where a client is OK with giving up the lock, and lock.allowLockRequest=false in the ones where it isn't--by default it's always false right now.

We should look at the situations where we might allow a lock to be released early.