Set the BlockHeader.first_transaction_consensus_time lazily

[Neeharika-Sompalli]

Currently BlockHeader. first_transaction_consensus_time is set in startRound. It looks only at the uiser transactions in the Round and sets the field.

But that field should represent the first transaction in the block, which could be a preceding child transaction or TSS transaction or staking transaction.

Solution:

• Set the field lazily after the first transaction that will be written to the block is decided

Testing:

• Unit tests
• Improve validator to see if this value is set on each block that has user transactions. Also validate if that is same timestamp as the first transaction inside the block