Rework write/sync ordering

Make syncs wait for all previous writes because it's the only way
to make sure that OSDs do not receive incomplete writes in LIST results
during peering when some writes are still in progress.

Also simplify blockstore submission queue logic.

src/blockstore_impl.h

@@ -160,8 +160,6 @@ struct blockstore_op_private_t
     // Sync
     std::vector<obj_ver_id> sync_big_writes, sync_small_writes;
     int sync_small_checked, sync_big_checked;
-    std::list<blockstore_op_t*>::iterator in_progress_ptr;
-    int prev_sync_count;
 };
 
 // https://github.com/algorithm-ninja/cpp-btree
@@ -212,7 +210,6 @@ class blockstore_impl_t
     blockstore_dirty_db_t dirty_db;
     std::list<blockstore_op_t*> submit_queue; // FIXME: funny thing is that vector is better here
     std::vector<obj_ver_id> unsynced_big_writes, unsynced_small_writes;
-    std::list<blockstore_op_t*> in_progress_syncs; // ...and probably here, too
     allocator *data_alloc = NULL;
     uint8_t *zero_object;
 
@@ -279,11 +276,9 @@ class blockstore_impl_t
     void handle_write_event(ring_data_t *data, blockstore_op_t *op);
 
     // Sync
-    int dequeue_sync(blockstore_op_t *op);
+    int continue_sync(blockstore_op_t *op, bool queue_has_in_progress_sync);
     void handle_sync_event(ring_data_t *data, blockstore_op_t *op);
-    int continue_sync(blockstore_op_t *op);
-    void ack_one_sync(blockstore_op_t *op);
-    int ack_sync(blockstore_op_t *op);
+    void ack_sync(blockstore_op_t *op);
 
     // Stabilize
     int dequeue_stable(blockstore_op_t *op);