Converted link_poll_end from global to instance variable.
Fixes likely bug when multiple work_queue instances are present.
Converted short_timeout from global into an instance variable.
Moved selection of timeout into send_worker_msg for consistency and clarity.
Updated comment: catalog details have been fixed, code has been released.
Merge branch 'work-queue-global-cleanup' of git://github.com/dthain/cctools into dthain-work-queue-global-cleanup

Removed last two global variables in work_queue.c and converted into instance
variables.  Moved computation of message timeout into send_worker_msg for
consistency and clarity.