Add support for parallel HTTP transfers

Add support for parallel HTTP transfers

Add support for parallel HTTP transfers.  Prefetch populates a queue of
objects to transfer and starts feeding requests to an active request
queue for processing; fetch_object keeps the active queue moving
while the specified object is being transferred.  The size of the active
queue can be restricted using -r and defaults to 5 concurrent transfers.
Requests for objects that are not prefetched are also processed via the
active queue.

Signed-off-by: Nick Hengeveld <nickh@reactrix.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>