stateManager: add modifyAccountFields method

[s1na]

Discussing the API with @jochem-brouwer we thought it might be easier from a developer PoV to have a modifyAccountFields that internally fetches an account from the trie, modify the given fields and then do a putAccount. This is to avoid users having to to the getAccount everytime they want to change an account. The method should probably throw if account doesn't exist.

[holgerd77]

This is still open (I would also say still useful?) and a potentially good first issue for some trie-releated StateManager/VM work.

Please assign yourself if you want to take and also add a test case testing the new method.

[emersonmacro]

I can try to tackle this