internal/charmstore: do not use extra-info for unit and machine counts
internal/charmstore: minor changes for review
Merge pull request #93 from rogpeppe/038-move-counts

internal/charmstore: do not use extra-info for unit and machine counts

As discussed online, extra-info is not an appropriate place to store data
generated by the charm store itself. This PR changes the existing code
to store the counts directly in the entity document. A subsequent PR
will add the bundle-unit-count and bundle-machine-count endpoints
for retrieval of the counts.
internal/v4: implement meta/bundle-unit-count and meta/bundle-machine-count
Merge pull request #94 from rogpeppe/039-count-endpoints

internal/v4: implement meta/bundle-unit-count and meta/bundle-machine-count

As specified in the charm API document.