The fun part was making it move things from TransferQueue to currentTransfers
entirely atomically. Which will avoid inconsistent display if the WebApp
renders the current status at just the wrong time. STM to the rescue!
I've convinced myself that nothing in DaemonStatus can deadlock,
as it always keepts the TMVar full. That was the only reason it was in the
Annex monad.
This should fix OSX/BSD issues with not noticing transfer information
files with kqueue. Now that threads are used, the thread can manage the
transfer slot allocation and deallocation by itself; much cleaner.
Check first if a transfer needs to be done, using the location log only
(for speed), and avoid occupying a slot if not. Always write a transfer
info file, and keep it open throughout the tranfer process.
Now transfers to remotes seem reliable.
There's still a bug; if the child updates its transfer info file,
then the data from it will superscede the TransferInfo, losing the
info that we should wait on this child.
