Fix thread safety

[rhaschke]

Problem:
Using process(...) within an event handler triggered from multiple threads in parallel causes a segfault, because the access to the root machine's process_ queue was unprotected. The problem is illustrated in a new unit test.

Solution:
I moved the mutex lock from some very deep process_event_impl functions to the top-level process_event.
This solution also eliminates the need for a recursive_mutex when using process.

[krzysztof-jusiak]

Awesome, thank you @rhaschke

[rhaschke]

@krzysztof-jusiak, this was a really fast merge!
What is your take on #562 (comment), which is another pressing issue for me.