Should work (untested) for transfers being run by other processes.
Not yet by transfers being run by the assistant. killThread does not
kill processes forked off by a thread. To fix this, will probably
need to make `git annex getkey` and `git annex sendkey` commands that
operate on keys, and write their own transfer info. Then the assistant
can run them, and kill them, as needed.
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.
