implement meta/revision-info

This is very close to the same as ExtendId which was just implemented.

The revision-info path returns information about other available revisions of the charm id that the charmstore knows about. It will include both older and newer revisions. The fully qualified ids of those charms will be returned in an ordered list from newest to oldest revision. Note that the current revision will be included in the list as it is also an available revision.
This isn't quite the right name for the type, as it's sorting by revision only.
This is quite a specialised requirement - please put it close to where
it's used, in the v4 package, and unexported.