commitlog: Discard active but unused segments on shutdown

So that they are not left on disk even though we did a clean shutdown.

First part of the fix is to ensure that closed segments are recognized
as not allocating (_closed flag). Not doing this prevents them from
being collected by discard_unused_segments(). Second part is to
actually call discard_unused_segments() on shutdown after all segments
were shut down, so that those whose position are cleared can be
removed.

Fixes #2550.

Message-Id: <1499358825-17855-1-git-send-email-tgrabiec@scylladb.com>
(cherry picked from commit 6555a2f50b)

db/commitlog/commitlog.cc

@@ -511,6 +511,7 @@ public:
         if (shutdown) {
             auto me = shared_from_this();
             return _gate.close().then([me] {
+                me->_closed = true;
                 return me->sync().finally([me] {
                     // When we get here, nothing should add ops,
                     // and we should have waited out all pending.
@@ -1319,6 +1320,7 @@ future<> db::commitlog::segment_manager::shutdown() {
                 return _gate.close().then(std::bind(&segment_manager::sync_all_segments, this, true));
             });
         }).finally([this] {
+            discard_unused_segments();
             // Now that the gate is closed and requests completed we are sure nobody else will pop()
             return clear_reserve_segments().finally([this] {
                 return std::move(_reserve_replenisher).then_wrapped([this] (auto f) {