cancelling requests due to max_request_time

[Frank Cusack]

request_list.c around line 574, calling pthread_cancel() on the thread
handling a timed out request is a bad idea.  It can leave resources
hanging, like a locked mutex for ownership of an ldap file descriptor.

In order for pthread_cancel() to be a good idea, most modules would
need to take care to disable cancellation while they hold resources
open, and/or set cancellation handlers to clean them up.  That's a
lot of work and there's a lot of room for bugs there.

But I can't think of a better approach.  You don't want to let threads
just run to completion and waste resources.

-frank