midlayer: VL server querying/caching services

midlayer: VL server querying/caching services

Add a couple of layers for VLDB querying and management.

The lower layer wraps collections of similar VL querying RPC calls, that
work out which variant to call and extract the retrieved data out into a
common data type.  Such functions include:

	kafs_VL_GetAddrs_by_uuid()
	kafs_VL_GetAddrs_by_addr()
	kafs_VL_GetAddrs_by_index()
	kafs_VL_GetEntryByName()
	kafs_VL_ListAttributes()

On top of that is a middle layer that may call sequences of the above and
add caching of the results:

	kafs_open_vl_service()
	kafs_probe_vl_service()
	kafs_look_up_volume_by_name()
	kafs_look_up_volumes_by_attributes()
	kafs_look_up_fileserver_by_uuid()
	kafs_look_up_fileserver_by_addr()
	kafs_look_up_fileserver_by_index()
	kafs_map_volume_locations()

Signed-off-by: David Howells <dhowells@redhat.com>