Fix row_cache::update

Clear continuous flag on the last cache entry
with key smaller than a partition being dropped from
memtable on flush and not saved in cache.

Signed-off-by: Piotr Jastrzebski <piotr@scylladb.com>
Message-Id: <0b5293cc0bf8bb858e62aa8dd00ae7fe7a484380.1467059472.git.piotr@scylladb.com>

row_cache.cc

@@ -757,6 +757,7 @@ future<> row_cache::update(memtable& m, partition_presence_checker presence_chec
                                 _tracker.insert(*entry);
                                 _partitions.insert(cache_i, *entry);
                             } else {
+                                --cache_i;
                                 cache_i->set_continuous(false);
                             }
                             i = m.partitions.erase(i);