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)

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

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

Checklist

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