Allow fractional relocking

[p-offtermatt]

Right now, it's possible to just refresh an existing lock. However, users may want to lengthen the lock only for part of their tokens, and may want the other tokens to stay locked with the existing lock duration.

This need seems best served by a new parameter in RefreshLockDuration, like AmountToKeepInOldLock. If this is 0, the behaviour is as-now when refreshing locks. If it is >total_locked_in_this_lock, the lock will stay unaffected.
Otherwise, there will be two locks, the old lock with AmountToKeepInOldLock tokens left, and a new lock with the remaining tokens.