Storage policy cleaned, storage class explicit loading

Now able to load a storage class explicitely, out of any storage policy context.

This is necessary for the metacd-http to process load-balancing requests with stgclass constraints. In facts, the metacd_http simply wraps the calls to metautils/lib/lb.h API.

Also, stgpol-related structures have been exposed in a nearly-hidden header, for testing purposes.
A corner case where a storage policy should exist but have no fallback has been fixed in the gridcluster configuration.
Finally, functions have been reordered "by purpose".

Change-Id: Ibd66594d53666f5ca4d1fab6d4226e1dc17ba95d