Makes w.cur into w.minrev, the minimum revision for the next update, and
retries cancelation if the watcher isn't found (because it's being processed
by moveVictims).
Fixes: #5459
Instead of holding the store lock while doing a lot of work like when syncung
unsynced watchers, the work from a blocked synced notify can be reused and
dispatched without holding the store lock for long.
Migrate command accepts a datadir and an optional user-provided
transformer function that transform v2 keys to v2 keys.
Migrate command then builds a v3 backend state based on the existing
v2 keys and the output of the transformer function.
