Rework VSL_Dispatch

Highlights:

* Improved buffer handling. A vtx can now have a mix of buffered and
  shm chunks. And as shm refs are used and later buffered, they can be
  reused on the same vtx. This benefits long running transactions.

* Synth records are now returned by the cursor at the position the
  cursor was at when it was created. This solves a problem when
  writing to a file and a vtx times out, the reading of that file
  would also cause a time out delay.

* Begin records are now strictly honored, and any records before a
  Begin record are now ignored.

* More asserts on the structures

* More comments, should be easier to understand now

* Various other changes