When using git-annex get -J2, one thread can clear another thread's
transfer lock.

This happens because fcntl locking is used, and it is not thread-safe.
<https://github.com/haskell/unix/issues/44>

>        *  If a process closes any  file  descriptor  referring  to  a
>           file,  then  all  of  the  process's locks on that file are
>           released, regardless of the file descriptor(s) on which the
>           locks  were obtained. 

One thread starts a transfer and locks it; the second thread starts a
transfer of a different file, and in order to check annex.diskreserve,
checks to see which other transfers are currently running. In doing this
check, it closes a fd attached to the first thread's lock, which causes the
lock to be dropped.

This only affects -J mode.

To fix it, probably need to use STM to keep a list of transfers all threads
in the current process are doing. Then the lock checking code can avoid
re-opening locks for transfers in the STM list.

A more generic approach is to use Annex.LockFile for everything,
and make it check its lock pool via STM for other threads holding a lock.
(Currently, the pool is only used for shared locks.)
--[[Joey]] 

> [[fixed|done]] via LockPools. --[[Joey]]
