Branch b1_8_gate

Branch b1_8_gate
b=16129
i=adilger
i=green

- a high priority request list is added into service;
- once a lock is canceled, all the IO requests, including coming
  ones, under this lock, are moved into this list;
- PING is also added into this list;
- once a lock cancel timeout occurs, the timeout is prolonged
  if there is an IO rpc under this lock;
- another request list is added into the export, used to speed up
  the rpc-lock matching.