Introduce the `timestamp` field in `gc_checkpoint` so now `gc.{c,h}` are
aware of actual times of checkpoints, which is important since this
subsystem is responsible for scheduling. Now, one can track the unix time
of a new checkpoint with new `timestamp` argument of `gc_add_checkpoint`.
This change allows us to track previous checkpoints made before the
server restart and even `checkpoint_interval` value reconfiguring.

This approach was chosen instead of just scanning the `snap_dir` in
`gc.c` because it was engine-independent. One may also notice that even
if the actual time after the last snapshot before the restart is greater
than `2 * checkpoint_interval` we won't start checkpointing immediately
because that may cause high disk load in case of multiple instances. So
in this case we just schedule a checkpoint at a random moment in the
first `checkpoint_interval` seconds after the restart. It seems like
even with this scheduling strategy a snapshot will be eventually created
even during constant restarting.

Fixes tarantool#9820
NO_DOC=bugfix