Make UnmanagedTransaction return ongoing tx completion stage (neo4j#1057)

This update ensures that `UnmanagedTransaction` returns existing on-going tx completion stage when a similar request is made. For instance, if it was requested to be rolled back and then requested to be closed, both invocations should get the same on-going stage. In addition, it should not accept conflicting actions, like committing and rolling back at the same time.

In addition, it makes sure that cancellation on reactive transaction function results in rollback.

Author: injectives