Ping-related improvements

[palkan]

What is the purpose of this pull request?

This PRs combines several improvements regarding server-client pings and session configuration.

What changes did you make? (overview)

• Added ability to configure ping interval and precision per client.
• Refactor setting protocol-specific session options
  • Do not store sessions not using the extended protocol in cache
  • Move previous session ID extraction from session.go to session_options.go (session shouldn't care about where the ID is coming from)
  • Extract protocol-specific session configuration (that would allow re-using the same endpoint for different protocols based on sub-protocol)
• Add small jitter to pings (so they're not happening simultaneously when tons of clients connecting at the same time)
• Add pong-s support (when the extended protocol is used)

Is there anything you'd like reviewers to focus on?

Checklist

• I've added tests for this change
• I've added a Changelog entry
• I've updated documentation