* Implement first response parts of http caching
* Implement cached response for static resources
* Implement HTTP caching for json responses
* Fix last seen value for online players
* Implement http caching for pages (.html)
* Use placeholder cache even with async requests.
Affects issues:
- Close#2813
- Split Session into ActiveSession and FinishedSession, replaced their usage
- Replaced UUID with ServerUUID when the data type
Affects issues:
- Close#1746