db/batchlog_manager: implement cleanup after all batchlog replay

We have a commented code snippet from Origin with cleanup and a FIXME to
implement it. Origin flushes the memtables and kicks a compaction. We
only implement the flush here -- the flush will trigger a compaction
check and we leave it up to the compaction manager to decide when a
compaction is worthwhile.
This method used to be called only from unbootstrap, so a cleanup was
not really needed. Now it is also called at the end of repair, if the
table is using repair-based tombstone-gc. If the memtable is filled with
tombstones, this can add a lot of time to the runtime of each repair. So
flush the memtable at the end, so the tombstones can be purged (they
aren't purged from memtables yet).

db/batchlog_manager.cc

@@ -284,19 +284,10 @@ future<> db::batchlog_manager::replay_all_failed_batches() {
                     });
                 });
             });
-        }).then([] {
-        // TODO FIXME : cleanup()
-#if 0
-            ColumnFamilyStore cfs = Keyspace.open(SystemKeyspace.NAME).getColumnFamilyStore(SystemKeyspace.BATCHLOG);
-            cfs.forceBlockingFlush();
-            Collection<Descriptor> descriptors = new ArrayList<>();
-            for (SSTableReader sstr : cfs.getSSTables())
-            descriptors.add(sstr.descriptor);
-            if (!descriptors.isEmpty()) // don't pollute the logs if there is nothing to compact.
-            CompactionManager.instance.submitUserDefined(cfs, descriptors, Integer.MAX_VALUE).get();
-
-#endif
-
+        }).then([this] {
+            // Replaying batches could have generated tombstones, flush to disk,
+            // where they can be compacted away.
+            return replica::database::flush_table_on_all_shards(_qp.proxy().get_db(), system_keyspace::NAME, system_keyspace::BATCHLOG);
         }).then([] {
             blogger.debug("Finished replayAllFailedBatches");
         });