For asynchroneous calls, the callbacks are performed on the main event loop
while the worker thread is blocked on a mutex.  At the moment, only boolean,
numeric and string values can be passed between the XSLT processor and the
JavaScript function.  Other types would be highly desirable, but might
require a lot more work.

This ensures that the main event loop will terminate without a call to
process.exit.  And it satisfies the requirement from the manual that the
async must be closed before being deallocated.  Closing on the v8 thread
avoids an abort when v8 itself shuts down, details unknown.

The core idea is that we allocate the context before we apply the
stylesheet.  That way we can store task-specific stuff in that context, like
the information whether we are single- or multi-threaded.  We might also be
storing per-context functions there.