row_cache: Introduce row_cache::touch()

Useful in tests for ensuring that certain entries survive eviction.

row_cache.cc

@@ -196,6 +196,13 @@ future<> row_cache::update(memtable& m, partition_presence_checker presence_chec
     });
 }
 
+void row_cache::touch(const dht::decorated_key& dk) {
+    auto i = _partitions.find(dk, cache_entry::compare(_schema));
+    if (i != _partitions.end()) {
+        _tracker.touch(*i);
+    }
+}
+
 row_cache::row_cache(schema_ptr s, mutation_source fallback_factory, cache_tracker& tracker)
     : _tracker(tracker)
     , _schema(std::move(s))

row_cache.hh

@@ -150,6 +150,10 @@ public:
     // The memtable can be queried during the process, but must not be written.
     // After the update is complete, memtable is empty.
     future<> update(memtable&, partition_presence_checker underlying_negative);
+
+    // Moves given partition to the front of LRU if present in cache.
+    void touch(const dht::decorated_key&);
+
     auto num_entries() const {
         return _partitions.size();
     }