Header files are now self-contained, which makes the relationships
between the files clearer, and crucially allows LSP tools like `clangd`
to function correctly in all of our header files. I have verified that
the headers are self-contained (aside from the various Windows shims) by
compiling them as if they were C files – in a follow-up commit I plan to
add this to CI to ensure we don't regress on this front.
Previously the calculation of sleep time between wakeups was implemented within
background_thread. This resulted in some parts of decay and hpa specific
logic mixing with background thread implementation. In this change, background
thread delegates this calculation to arena and it, in turn, delegates it to PAI.
The next step is to implement the actual calculation of time until deferred work
in HPA.
