Patch adds class variable 'timeout' to BaseServer, and
methods await_request and handle_timeout.  Backwards
compatible API -- timeout is None by default, meaning
await_request jumps directly to get_request and never
calls handle_timeout.

Facilitates periodic server bookkeeping (check config
files, intervalic logging, etc.), if you supply your
own serve loop.

Additionally, ForkingMixin sets timeout to 300 seconds,
and overrides handle_timeout to call collect_children.
 Upshot:  n zombies linger in process table until 5
minutes pass or new request arrives, whichever comes first.

docstring updates, too
(s/reuse_address/allow_reuse_address, etc.)

-Mike