Since the lock file has to be kept open, this prevented the TransferWatcher
from noticing when it appeared, since inotify (and more importantly kqueue)
events happen when a new file is closed. Writing a separate info file fixes
that problem.
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.
Not yet tested and places git-annex-shell is run need to be modified to
pass the new field settings.
Note that rsyncServerSend was changed to fork, rather than directly exec
rsync, because it needs to keep the transfer lock held, and clean up the
transfer log when done.
In order to record a semi-useful filename associated with the key,
this required plumbing the filename all the way through to the remotes'
storeKey and retrieveKeyFile.
Note that there is potential for deadlock here, narrowly avoided.
Suppose the repos are A and B. A sends file foo to B, and at the same
time, B gets file foo from A. So, A locks its upload transfer info file,
and then locks B's download transfer info file. At the same time,
B is taking the two locks in the opposite order. This is only not a
deadlock because the lock code does not wait, and aborts. So one of A or
B's transfers will be aborted and the other transfer will continue.
Whew!
