Implement LV2 worker extension.

Implement LV2 worker extension.

This is done by way of a generic Worker object/thread, which currently just
applies to one LV2 plugin, but the idea is to share one thread and set of
buffers among many plugins.  The same pattern may also be useful elsewhere in
Ardour.  The responding part gets a bit tricker when sharing a worker between
plugins, it's not a blocker, and I'm lazy, sooo here's this.

This commit also adds a new portable in-process semaphore to PBD.  The existing
one is pretty weird and uses a named semaphore on OSX for reasons unknown to
me.  Perhaps as a quick fix to avoid POSIX semaphores being utterly broken on
OSX?  It would probably be a good idea to replace that with this new one, which
uses Mach kernel semaphores on OSX which work well, though I am not sure how
pedantically real-time safe they are to signal.

git-svn-id: svn://localhost/ardour2/branches/3.0@11790 d708f5d6-7413-0410-9779-e7cbd77b26cf