refactor(store)!: change Store's method names

The changes in this commit were motivated due to a bug in the
`StoreFile` which caused old data to be lost if the file was `open`
instead of created and new data was appended.
The bugfix later motivated a general name cleanup in StoreFile's methods
and some minor changes in their signatures. The new StoreFile methods
are:
- create: create file in write only mode or fail if file exists.
- load: open existing file, check integrity of content and retrieve
  Store.
- append: add new changesets to Store. Do nothing if changeset is empty.
- dump: aggregate and retrieve all stored changesets in store.
- load_or_create: load if file exists, create if not, and retrieve
  Store.

Author: nymius