fix for attaching a context and clearin reversible blocks

There was a race between attaching context and removing reversible blocks. Very rarely, during the process of attaching context, an irreversible block was set based on already synchronized irreversible blocks. However, if a new irreversible block was reported by hived between computing the new irreversible block and updating the context, the context might search for a reversible block that had already been removed.

test_application_broken.py: from now there is no guarantee that obsolete
reversible blocks will be removed with each calling hive.set_irreversible