Denormalize entity ids on Mongo.

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