lease Manager should not evaluate all Leases and all Blocks on every loop()

worker/lease/manager.Manager.loop() currently evaluates all Leases 2x in every step of loop().

Once in nextTick() to figure out when the next tick would be and once in loop() where it checks if there are any blocks to unblock.

However, we should only need to unblock if something expired, and we can compute nextTick once when we evaluate the leases, and then only tick the clock forward a bit if a ClaimLease would cause the most-recent expiry to be newer than the previous most recent.