Extending threadIsMain() assert

[SirTyson]

Given the amount of parallelism being added to core with #4543, we now have many different types of threads responsible for different tasks. Binary asserts of !threadIsMain() and threadIsMain() no longer hold as much meaning, and we've had to loosen our thread asserts in a lot of places. We should extend the threadIsMain assert to all different "thread types," such as overlay thread, apply thread, etc. This shouldn't be too difficult. For each thread type we cal declare a 0 initialized thread::id like we currently do for mainThread, then we can initialize the value when spinning up the thread. It may also be useful to rename the main thread to something like "consensus thread" or "scheduling thread" so it carries a little more meaning.