Fix logic race in chttp2 write path

IF:
- we schedule a write in chttp2 in response to some stream op
  (which will cause a write that's covered by a poller to be scheduled
  on the combiner lock)
- AND then, under that same combiner lock, we process a RST_STREAM
- then we'll remove the op that's being processed, consequently removing
  the polling coverage
- and then, IF that is the last poll on said transport, the transport
  will never write, which CAN cause servers to fail to shutdown