Fix bufferevent_pair to properly set BEV_EVENT_{READING,WRITING} on flush.

Fix bufferevent_pair to properly set BEV_EVENT_{READING,WRITING} on flush.

Here's some fun. From `bufferevent.h`:
  ```
  #define BEV_EVENT_READING	0x01	/**< error encountered while reading */
  #define BEV_EVENT_WRITING	0x02	/**< error encountered while writing */
  ```

And from `event.h`:
  ```
  /** Wait for a socket or FD to become readable */
  #define EV_READ		0x02
  /** Wait for a socket or FD to become writeable */
  #define EV_WRITE	0x04
  ```

Library users have to be very careful to get this right; it turns out, the
library itself got this wrong in the `bufferevent_pair` code. It appears that
in most of the code, only `BEV_EVENT_FINISHED` will indicate whether it's read
or write; on error or timeout, it appears that "both" is assumed and not set in
the callback. I read through all the other places where `BEV_EVENT_FINISHED` is
passed to an event callback; it appears that the pair code is the only spot
that got it wrong.

azat: add TT_FORK to avoid breaking clean env, and rebase commit message
(copied from #359)
Fixes: #359