Require less `mut`, allow multiple (non-mut) references to Client

[ctron]

trafficstars

The function execute_cypher of Client requires &mut self, which doesn't seem to be required. However, having it mut seems to make it impossible to use transactions, as that would also require &mut self, which flows into the transaction.

[Dzordzu]

I'll look into that in the following week. Thanks for reporting!

[ctron]

It was easy to come up with a PR, so here is #12 .