poll-loop: Simplify and speed up polling.

poll-loop: Simplify and speed up polling.

The simplification comes from dropping support for canceling a
poll_waiter, which was a feature that was never used.  The speedup
comes from avoiding a malloc() for every call to poll_fd_wait().
(I doubt that this significantly improves performance.)

This prepares for making the polling structures per-thread in
the next commit.

Signed-off-by: Ben Pfaff <blp@nicira.com>