commitlog: update last sync timestamp when cycle a buffer

If in memory buffer has not enough space for incoming mutation it is
written into a file, but the code missed updating timestamp of a last
sync, so we may sync to often.
Message-Id: <20200102155049.21291-9-gleb@scylladb.com>

db/commitlog/commitlog.cc

@@ -852,7 +852,7 @@ public:
                 // If we run batch mode and find ourselves not fit in a non-empty
                 // buffer, we must force a cycle and wait for it (to keep flush order)
                 // This will most likely cause parallel writes, and consecutive flushes.
-                return with_timeout(timeout, cycle(true)).then([this, id, writer = std::move(writer), permit = std::move(permit), timeout] (auto new_seg) mutable {
+                return with_timeout(timeout, sync()).then([this, id, writer = std::move(writer), permit = std::move(permit), timeout] (auto new_seg) mutable {
                     return new_seg->allocate(id, std::move(writer), std::move(permit), timeout);
                 });
             } else {