Eliminate need for an other parent in single node networks

There is a quirk in the consensus algorithm that requires events in a single node network to have a non-null other parent. To get around this, we set the other parent to the self parent, but this should be addressed so that it is not necessary. When this is fixed, the logic in event validation that checks for the other and self parents having the same hash should be simplified.

Start by tracing event creation and validation for a single-node network in the consensus implementation; the issue names the other-parent requirement and the validation check but no files or tests. Done means single-node events no longer need a non-null other parent, and event validation no longer needs the special same-hash check.

Written by the indexing model from the issue text.