After winning an election or obtaining a lock, we
auto-append a slash after the provided key prefix.
This avoids the previous deadlock due to waiting
on the wrong key.
Fixes#6278
With randomized keys, if the connection goes down, but the session remains,
the client would need complicated recovery logic to avoid deadlock.
Instead, bind the session's lease id to the lock entry; if a session tries
to reacquire the lock it will reassume its old place in the wait list.
Lock would get the prior key on retry using WithRev(myRev - 1) instead of
using the latest revision; the Watch() would return immediately and Lock
devolves into polling.
