importers: improve apkindex import

Improve performance of apkindex import by only commit db changes after the entire index is imported instead of committing each package.

Before:

  real	6m 7.64s
  user	3m 16.86s
  sys	0m 29.30s

After:

  real	0m 40.89s
  user	0m 39.08s
  sys	0m 0.22s

Also add optional repo argument so we can update a given repo instead of all. This is useful when connecting to a MQTT event for uploading to a given branch/repo.