Add KeyValueStore::execute.

[partim]

This PR adds a method KeyValueStore::execute that runs a closure as a transaction but allowing it to return a closure.

[partim]

This method could become KeyValueStore::transaction if drop the impl KeyValueStoreBackend for KeyValueStore. I think it would, in fact, make sense to drop all the backend impls for it and move to regular methods. If nothing else this will save you from having to import those traits. I don’t think someone wants to write code that is generic over a backend but also needs to accept the ‘full store’?

[timbru]

This will help me to achieve the following:

• use Result<MyValue, MyError> for T
• read current data (using a lock or transaction)
• do business logic, and exit with my own error type in case of issues
• write updates
• exit with kvx errors in case there are storage issues (this indicates potentially fatal issues)
• exit with my value if everything is okay

Calling it execute rather than transaction is probably better because we do not have proper transactions in all implementations.