Implemented the requirement "Intelligently retry if a host doesn't reply: Try it...

Implemented the requirement "Intelligently retry if a host doesn't reply: Try it more often"
- PingAnalyzer::exceeded_ping_failed_count_limit() made public, thus the Analyzer can check when is required to speed up the ping (or back to the original interval)
- Included a PingInterval template class to hide the computation logic of interval speed up and slow down