ManagedNewTransactionRunner is gettings some improvements:

• FluentFuture instead of ListenableFuture
• ManagedTransactionChain
• only-commit-if-written
• internal code duplication reduction

skitt initiated this work, I am helping to review and complement with some extensions.

We're creating this bug to keep track of it all - and make sure that (eventually) all related Gerrits also make it back from master to stable/fluorine and stable/oxygen, so that application changes relying on this can be back-ported as well. (jhershbe originally brought this up; he's working on something where he'd like to use at least the FluentFuture instead of ListenableFuture in Oxygen.)