gc: randomie the next checkpoint time also after a manual box.snapshot().

Before this patch, snapshot interval was set randomly within
checkpoint_interval period. However, after box.snapshot(), the next
snapshot was scheduled exactly checkpoint_interval from the current time.
Many orchestration scripts snapshot entire cluster right after deployment,
to take a backup. This kills randomness, since all instances begin to
count the next checkpoint time from the current time.

Randomize the next checkpoint time after a manual snapshot as well.

Fixes gh-4432