Implemented closure parsing and loading for Volume-wide closures.
Volume-wide closures implement the connect_cache() method, and do not
have secrets.  This is meant ot allow the user to deploy a Volume-wide
cache driver, independent of the gateway implementation (i.e. the
volume-wide closure is handled by libsyndicate).

I also added a stub Volume closure for CoBlitz
(libsyndicate/drivers/coblitz).  This commit also includes bugfixes to
syntool.py's parsers (ms/common/object_stub.py) to check the validity of
a closure and serialize it properly.