Allocate a dedicated thread to detect deadlocks in the runtime system

[rahulmutt]

In certain extreme cases, it is possible for the runtime system to deadlock and lightweight threads to stall. An example of an extreme case is when all of the runtime system allocated threads are all blocked on a Java FFI call. In such a case, it is beneficial to have a separate, dedicated thread that monitors the progress of lightweight threads and will spawn new heavy threads as required if no progress is being made.

Note that this is mainly hypothetical - we have not yet observed this in practice.