db/hints: Introduce an error injection to test draining

We want to verify that a hint directory is drained
when any of the nodes correspodning to it leaves
the cluster. The test scenario should happen before
the whole cluster has been migrated to
the host-ID-based hinted handoff, so when we still
rely on the mappings between hint endpoint managers
and the hint directories managed by them.

To make such a test possible, in these changes we
introduce an error injection rejecting incoming
hints. We want to test a scenario when:

1. hints are saved towards a given node -- node N1,
2. N1 changes its IP to a different one,
3. some other node -- node N2 -- changes its IP
   to the original IP of N1,
4. hints are saved towards N2 and they are stored
   in the same directory as the hints saved towards
   N1 before,
5. we start draining N2.

Because at some point N2 needs to be stopped,
it may happen that some mutations towards
a distributed system table generate a hint
to N2 BEFORE it has finished changing its IP,
effectively creating another hint directory
where ALL of the hints towards the node
will be stored from there on. That would disturb
the test scenario. Hence, this error injection is
necessary to ensure that all of the steps in the
test proceed as expected.

db/hints/manager.cc

@@ -409,6 +409,12 @@ bool manager::have_ep_manager(const std::variant<locator::host_id, gms::inet_add
 bool manager::store_hint(endpoint_id host_id, gms::inet_address ip, schema_ptr s, lw_shared_ptr<const frozen_mutation> fm,
         tracing::trace_state_ptr tr_state) noexcept
 {
+    if (utils::get_local_injector().enter("reject_incoming_hints")) {
+        manager_logger.debug("Rejecting a hint to {} / {} due to an error injection", host_id, ip);
+        ++_stats.dropped;
+        return false;
+    }
+
     if (stopping() || draining_all() || !started() || !can_hint_for(host_id)) {
         manager_logger.trace("Can't store a hint to {}", host_id);
         ++_stats.dropped;