Abort traffic replay if servers are unresponsive

If the target server doesn't send any reply within the configured time limit (when some are expected), abort the traffic replay. This most commonly happens due to either incorrect configuration or server crash. In the case of premature exit, ensure that existing statistics are collected.

The default (baked-in) server timeout limit is configured to be 16 seconds. This has proven to be the right magic number in the development of related tools (respdiff). It is unlikely that this limit would have to be reconfigured or disabled, so no way to adjust or turn off this limit is provided until a reasonable use-case is presented.

Closes #32