hip-739 Include Latest Consensus Timestamp in HAPI Responses

[nickpoorman]

Currently, when a user submits a transaction on the Hedera API (HAPI), they supply two properties: a consensus timestamp at which the transaction will be valid transactionID.transactionValidStart and a duration for which the transaction remains valid as transactionBody.transactionValidDuration. For users to provide an accurate consensus timestamp for transactionValidStart, they need to be aware of the latest consensus timestamp of the network. Also, when a user resubmits a transaction to the network, it may be necessary under some circumstances for the user to have the latest consensus timestamp of the network.

See https://github.com/hashgraph/hedera-improvement-proposal/pull/739/files