Add a DSL word that runs when a specific pool or all pools are idle

[TrentHouliston]

Adds a new DSL word Idle that can be used to execute a reaction when the pool does not have a current task.

There are 4 main variants that this implements

on<Idle<>>

Will execute when all non always threads are idle

on<Idle<Pool<>>

Will execute when the default thread pool is idle

on<Idle<MainThread>>

Will execute when the main thread's pool is idle

on<Idle<SomePoolDescriptor>>

Will execute when that specific pool is idle

• Fix up the deadlock caused by the recursive mutex acquisition