atomic wallet writes

[tzemanovic]

Change the API of the wallet so that instead of save method, the modification methods are atomic. We can load the wallet file with a lock on modifications (the lock's scope should be as small as possible), make the modification, save to file and release the lock. This is to allow avoid data-races from multiple processes

┆Issue is synchronized with this Asana task by Unito

[cwgoes]

@karbyshev can you take this on as part of the wallet work?