fix: allow event loop to process during wait queue processing

[mbroadst]

Running processWaitQueue on the next tick allows the event loop
to process while the connection pool is processing large numbers of
wait queue members. This also uncovered a few issues with timing
in our tests, and in some cases our top-level API:

• commitTransaction / abortTransaction use maybePromise now
• endSession must wait for all the machinery behind the scenes to
  check out a connection and write a message before considering its
  job finished
• internal calls to kill a cursor now await the the process of fully
  sending that command, even if they ignore the response

NODE-2803

NOTE: this is the port from 3.6 to 4.0

[reggi]

• One thought here is we may want to close the client regardless if success / error?
• If there's no reason to use the promise response do we need to call then? Or is there some sort of lingering promise issue?

[mbroadst]

My thought here was "its an empty iterator across the result set" so there would never be a failure, but just in case I switched it to just return the promise. The reason this was initially introduced is that the test was not actually ever waiting for the promise to resolve, so we had random errors because the internal logic was never able to take the time to check out a connection before the client was closed.