drain CQ after moving QP into RESET state

[live4thee]

Otherwise there might segfault due to the race below:

Socket::OnInputEvent()          |
  `-- ProcessEvent (bthread)    |
                                |
  [ bthread queueed ]           |  QP error -> SetFailed -> HC -> WaitAndReset()
                                |  Reset() -> _sbuf.clear()
                                |  CheckHealth() -> Revive()
                                |
                                |  Socket is now Addressable!
  RdmaEndpoint:PollCq()         |
  Socket::Address() OK!         |
  RdmaEndpoint:HandleCompletion()
  _sbuf[_sq_sent++].clear()    <= BOOM!  CQ is not drained but _sbuf is cleared.

Another possible fix is to add a generation field in RdmaEndpoint, such that:

• each RdmaEndpoint::Reset() will advance the generation by 1;
• the RdmaEndpoint::PollCq(m, orig_gen) will need to compare the generation.

But it will contaminate existing interface, and we need to drain CQ anyway.

What problem does this PR solve?

Issue Number: 3252

Problem Summary: see above.

What is changed and the side effects?

Changed:

1. drain CQ after moving QP into RESET state;
2. do not re-use QP if drain failed.

Side effects:

• Performance effects: n/a

• Breaking backward compatibility: none

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