database: do not use implicitly stated seal function when closing the CF

In commit 4981362f57, I have introduced a regression that was thankfully
caught by our dtest infrastructure.

That patch is a preparation patch for the active reclaim patchset that is to
come, and it consolidated all the flushes using the memtable_list's seal_fn
function instead of calling the seal function explicitly.

The problem here is that the streaming memtables have the delayed mechanism,
about which the memtable_list is unaware. Calling memtable_list's
seal_active_memtable() for the streaming memtables calls the delayed version,
that does not guarantee flush. If we're lucky, we will indeed flush after the
timer expires, but if we're not we'll just stop the CF with data not flushed.

There are two options to fix this: the first is to teach the memtable_list about
the delayed/forced mechanism, and the second is to just call the correct
function explicitly during shutdown, and then when the time comes to add
continuations to the result of the seal, add them here as well.

Although the second option involves a bit more work and duplication, I think it
is better in the sense that the delayed / forced mechanism really is something
that belong to the streaming only. Being this the only user, I don't think it
justifies complicating the memtable_list with this concept.

Signed-off-by: Glauber Costa <glauber@scylladb.com>
Message-Id: <b26017c825ccf585f39f58c4ab3787d78e551f5f.1464126884.git.glauber@scylladb.com>

database.cc

@@ -771,8 +771,8 @@ column_family::start() {
 
 future<>
 column_family::stop() {
-    _memtables->seal_active_memtable();
-    _streaming_memtables->seal_active_memtable();
+    seal_active_memtable();
+    seal_active_streaming_memtable();
     return _compaction_manager.remove(this).then([this] {
         // Nest, instead of using when_all, so we don't lose any exceptions.
         return _flush_queue->close().then([this] {