Eliminate TransactionContext.executeModification()

Refactor done in CONTROLLER-1950 makes it obvious that the internal interface between local, noop and remote contexts is needlessly allocating AbstractModifications.

Without the refactor, this actually hurts, because we are performing two allocations, which actually may not be properly escape-analyzed.

After the refactor the allocation short-lived, so can be inferred out in local case dominance – but it also means TransactionProxy knows about remoting objects. It clearly should not, as those objects should be allocated only in RemoteTransactionContext.