ENG-5442: Fix ElasticContext.handleActivation() validation.

If the context it's trying to activate is not the elastic context, then don't
bother validating. Otherwise, if there is a snapshot in progress, all
@BalancePartitions calls will fail even though they don't activate elastic
context.
Merge pull request #913 from VoltDB/ENG-5442

ENG-5442: Fix ElasticContext.handleActivation() validation.