Fix bug in StateMap::get_mut

[Bargsteen]

Purpose

This PR fixes an issue in StateMap::get_mut which allowed users to get inconsistent views of the data. There is an example below.

When using StateMap::get_mut a value is loaded from the persistent storage into memory. The in-memory value can then be mutated and it will be written to the persistent storage once the value is dropped (via the Drop implementation on StateRefMut).

If you are able to look up multiple mutable values from the state at once, you can get inconsistent views of the data.
Here is an example:

token_count has the in-memory value x + 2 and token_count has the value x as it is freshly loaded from persistence.

For this reason, we use lifetimes to ensure that you cannot have multiple mutable references to the state at the same time.
Or, at least that was what we meant to do!
We accidentally gave an immutable reference of self which exactly allows the example above to run instead of being caught by the borrow-checker.

Changes

• Add three letters: mut (and a space)
• Add proper tests using testbuild to ensure that it won't occur again

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[abizjak]

Good catch.