Add a git config to limit the bandwidth of transfers to/from remotes.

rsync has --bwlimit, so used to work, but is not used with modern git-annex for p2p transfers. (bup also has a --bwlimit)

This should be possible to implement in a way that works for any remote that streams to/from a bytestring, by just pausing for a fraction of a second when it's running too fast. The way the progress reporting interface works, it will probably work to put the delay in there. --Joey