Feature/cleanup api

Depends on: https://github.com/leapcode/leap_mail/pull/152
Interrelated with: https://github.com/leapcode/bitmask_client/pull/756 & https://github.com/leapcode/leap_pycommon/pull/92

_I'm not sure if commits should be rebased together_
bitmask_client is not using it, see the 'return' in front. I was adding a comment to it with my commit there. I didn't know about soledad, if soledad uses it I guess I'll have to revert the removal.

I don't like the functionality and name of this method, the rest of the class works as a key storage but this one does key parsing, that has nothing to do with the storage, it actually don't load the key in the storage. I'll give it more thought.