Cleanup and prepare for thread-safety changes.
Big ideas:
* Wrap H5VL_class_t with H5VL_connector_t, so use of the class can be refcounted within the H5VL package, instead of relying on storing an ID within the H5VL_t struct and incrementing & decrementing the ID's refcount.
* Register H5VL_connector_t* for VOL connector IDs, instead of the H5VL_class_t*
* Stop other packages from rummaging around inside H5VL_connector_t and H5VL_object_t data structures, so that the H5VL package can change implementation details without coupled changes throughout the library
Small things:
* Simplified the coding for creating links
* Moved some routines into more logical locations
