The repair_time in system.tablets will be updated when repair runs
successfully. We can now use it to update the repair time for tombstone
gc, i.e, when the system.tablets.repair_time is propagated, call
gc_state.update_repair_time() on the node that is the owner of the
tablet.
Since b3b3e880d3 ("repair: Reduce hints and batchlog flush"), the
repair time that could be used for tombstone gc might be smaller than
when the repair is started, so the actual repair time for tombstone gc
is returned by the repair rpc call from the repair master node.
Fixes#17507
53e6025aa6