querier_cache: unregister queriers evicted due to expired TTL

Currently queriers evicted due to their TTL expiring are not
unregistered from the `reader_concurrency_semaphore`. This can cause a
use-after-free when the semaphore tries to evict the same querier at
some later point in time, as the querier entry it has a pointer to is
now invalid.

Fix by unregistering the querier from the semaphore before destroying
the entry.

Refs: #4018
Refs: #4031

Signed-off-by: Botond Dénes <bdenes@scylladb.com>
Message-Id: <4adfd09f5af8a12d73c29d59407a789324cd3d01.1546504034.git.bdenes@scylladb.com>

querier.cc

@@ -191,6 +191,7 @@ void querier_cache::scan_cache_entries() {
     while (it != end && it->is_expired(now)) {
         ++_stats.time_based_evictions;
         --_stats.population;
+        _sem.unregister_inactive_read(it->get_inactive_handle());
         it = _entries.erase(it);
     }
 }