Refactored duplicate code into Transaction.setInitialApprovalStatus().
This is make sure transactions HAVE a group-approval status
in Synchronizer before Block.isValid is called.
This wasn't a problem for new, unconfirmed, individual transactions
arriving over the wire due to Transaction.importAsUnconfirmed()
doing the right thing.
Also added a groupApprovalTimestamp to BlockChain feature-triggers
to support legacy chains.
Tested by syncing mainnet from scratch.
