Add queue stall tracking

blockstore_impl.h

@@ -207,6 +207,7 @@ class blockstore_impl_t
     struct journal_t journal;
     journal_flusher_t *flusher;
 
+    bool live = false, queue_stall = false;
     ring_loop_t *ringloop;
 
     bool stop_sync_submitted;
@@ -281,6 +282,9 @@ public:
     // loop until it returns true.
     bool is_safe_to_stop();
 
+    // Returns true if stalled
+    bool is_stalled();
+
     // Submission
     void enqueue_op(blockstore_op_t *op);