lppublish, cmd/charmload: ingest exact revisions from legacy charm store
Merge pull request #128 from rogpeppe/058-use-legacy-revisions

lppublish, cmd/charmload: ingest exact revisions from legacy charm store

This behaviour is controlled by the new "-legacy" flag to charmload, which should
be used just once, when initially uploading charms.

In passing, also adjusted log messages somewhat so the logging at INFO
level is a little quieter and more informative.