This change replaces the barrier-style synchronization for diff
construction  and compression with a task pool. The collect_update
function now creates a series of tasks, which are run using the new
thread pool. (The main thread can also run tasks, which is helpful
when e.g. running with exactly one thread.) On completion, each task
inserts a message into the transfer queue, and notifies the main
loop poll by writing to a self-pipe. The main thread writes the new
transfer data to the channel fd as necessary. When all pool tasks are
complete, the protocol messages are appended to the transfer queue
and sent.