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]]

[[confirmed]]