LibBoostAsioHandler does not detect connection timeouts when heartbeat is negotiated

[ricardocrl]

Opposed to libev integration, libboostasio.h does not implement any mechanism to detect connection drops.

At first, I tried using TcpHandler::onHeartbeat() to detect timeouts, but this is not sufficient because the broker does not send heartbeats if there is data flowing to us. Intercepting these events on top of boost::asio integration is not trivial or beauty.

When comparing to libev.h, this is solved with onActive being called and extending the timeout timer. So I decided to fix this by porting the solution into libboostasio.h, hooking the timer update in the most equivalent method read_handler. This is the PR for the changes proposed: #463.

On a separate note, I believe this code can still be refactored for a better integration with boost. I'm not fluent with boost or boost::asio, but I may suggest some changes with care.