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.
We have just one service, so I believe this should be just "5".