[threading] Introduce the notion of try block to be used by recursive locks.

[threading] Introduce the notion of try block to be used by recursive locks.

This allows a piece of code to handle the cases where it's called in running and blocker context
and must always be in blocking context.

This probably makes code more modular at the expense of an easy to follow and debug execution model.

Since we don't currently annotate all icalls with blocking sections we do it with locks but they can
nest and recurse which requires this support for try block.