box.snapshot() should randomize next snapshot interval

[kostja]

In tarantool, snapshot interval is set randomly within checkpoint_interval period. However, if you do box.snapshot(), the next snapshot is scheduled exactly checkpoint_interval from the current time.
Many deployment systems snapshot entire cluster right after deployment, to take a backup. This kills randomness, since all instances begin to count next interval from the current time.

Fix: randomize the next checkpoint time after a snapshot.