With the introduction of batch/v1 and batch/v2alpha1, any
MultiRESTMapper that has per groupversion mappers (as a naive discovery
client would create) would end up being unable to call RESTMapping() on
batch.Jobs. As we finish up discovery we will need to be able to choose
prioritized RESTMappings based on the service discovery doc.

This change implements RESTMappings(groupversion) which returns all
possible RESTMappings for that kind.  That allows a higher level call to
prioritize the returned mappings by server or client preferred version.