hinted handoff: don't keep positions of old hints in rps_set

When sending hints from one file, rps_set field in send_one_file_ctx
keeps track of commitlog positions of hints that are being currently
sent, or have failed to be sent. At the end of the operation, if sending
of some hints failed, we will choose position of the earliest hint that
failed to be sent, and will retry sending that file later, starting from
that position. This position is stored in _last_not_complete_rp.

Usually, this set has a bounded size, because we impose a limit of at
most 128 hints being sent concurrently. Because we do not attempt to
send any more hints after a failure is detected, rps_set should not have
more than 128 elements at a time.

Due to a bug, commitlog positions of old hints (older than
gc_grace_seconds of the destination table) were inserted into rps_set
but not removed after checking their age. This could cause rps_set to
grow very large when replaying a file with old hints.

Moreover, if the file mixed expired and non-expired hints (which could
happen if it had hints to two tables with different gc_grace_seconds),
and sending of some non-expired hints failed, then positions of expired
hints could influence calculation _last_not_complete_rp, and more hints
than necessary would be resent on the next retry.

This simple patch removes commitlog position of a hint from rps_set when
it is detected to be too old.

Fixes #6422

db/hints/manager.cc

@@ -703,6 +703,7 @@ future<> manager::end_point_hints_manager::sender::send_one_hint(lw_shared_ptr<s
                 // Files are aggregated for at most manager::hints_timer_period therefore the oldest hint there is
                 // (last_modification - manager::hints_timer_period) old.
                 if (gc_clock::now().time_since_epoch() - secs_since_file_mod > gc_grace_sec - manager::hints_flush_period) {
+                    ctx_ptr->rps_set.erase(rp);
                     return make_ready_future<>();
                 }