Split timings into tree reports
this will allow the timings site to display timings as a tree, instead of as a list as is done now, which will enable more precise identification of performance issues.
An example of this can be seen here: https://timings.pmmp.io/?id=302629
The format changes are fully backwards compatible, as the timings site
aggregates timings from timers with the same names, and doesn't limit
how much extra data can appear at the end of a line.
this fixes#3906.
TimingsHandler will now be automatically destroyed when there are no TimingsRecord referencing it and its owner has no references left to it.
TimingsRecord will be automatically destroyed at the end of the timings session (if active).
This fixes records from temporary timers being lost after the timings handlers are removed, while also fixing TimingsHandlers piling up regardless of whether timings are running or not.
This opens the doors to have more volatile timers, such as per-player/per-session timers, which are destroyed on player quit, but which won't result in the timings disappearing from the final timings result.
