Denormalize entity ids on Mongo.

When adding charms or bundles, store their URL parts
in separate fields.
Also implement a simple migration strategy.
Add boosting of bundles.

Bundles are now boosted to be just above the highest series.
Improve the migrations infrastructure.
Improve migrations docstrings.
Merge pull request #237 from mhilton/bundle-boost

Add boosting of bundles.

Bundles are now boosted to be just above the highest series.
Update ES with the new denormalized fields.
Increment ES settings version number.
Improve migration logging and error messages.
Add a test exercising concurrent migrations.
Merge pull request #236 from frankban/denormalize-entity-ids

Denormalize entity ids in Mongo.

When adding charms or bundles, store their URL parts
in separate fields.
Also implement a simple migration strategy.