Aggregate default cannot be overridden by configuration

[bbengfort]

On config.go L31 - the default aggregate option is set to true, however, because of the way the config.Update method works on config.go L98 with the IsZero check, this cannot be set to false via an external configuration file. This needs to be rethought both here and in epaxos.

Note that the epaxos solution is to simply set aggregate to false as the default, then specify true as needed.