Great job.

However (as mentioned before), I believe the proper way of solving the issue is by implementing keepalive on top of the asio [1] pull request (that's what Jeff suggested). I've looked into the issue reported there - turns out to be a trivial fix (send buffer goes out of scope, hence large tx'es fail).

Implementing keepalive on top of that should only take a few lines (header handling and timeout), and is probably a better merge candidate. Seems to work for me after fixing the asio patch and adding a quick hack to accept additional requests.

[1]: https://github.com/bitcoin/bitcoin/pull/214