Implement simple "flow control" for RDMA (to check hypothesis about slowdowns)

src/msgr_rdma.h

@@ -45,7 +45,8 @@ struct msgr_rdma_connection_t
     ibv_qp *qp = NULL;
     msgr_rdma_address_t addr;
     int max_send = 0, max_recv = 0, max_sge = 0;
-    int cur_send = 0, cur_recv = 0;
+    int cur_send = 0, cur_recv = 0, avail_recv = 0, avail_send = 0;
+    bool use_flow_control = false;
     uint64_t max_msg = 0;
 
     int send_pos = 0, send_buf_pos = 0;