[Client Refactoring 2] Add a stateless version of execution_transaction

[lxfind]

To make it easier to move execution_transaction to AuthorityAggregator, we first introduce a stateless version of it, and call it from the stateful version. This stateless version will then latter be moved to AuthorityAggregator.
The reason I am doing it this way is to make PRs less intrusive / easier to review, and make the refactoring more smooth. It also allows other people to continue working on the client core.
Most of this PR is straightforwards as most functions called are already stateless.
The only thing I needed to change is to move around the update of local objects. There are two of them, once in the execution_transaction, while the other one in execute_transaction_without_confirmation. The one in execute_transaction_without_confirmation is unnecessary and can be removed.

[oxade]

@lxfind nice! Thanks for jumping on this.
You might want to look at some refactoring and modularization work @gdanezis is doing here in case of overlaps/conflicts.
I think there's a lot of similarities and I particular like the MapReduce view in George proposes.
#336

[gdanezis]

Yes, the authority API should indeed be useable with &refs, not &mut.