Store FullTransactionId in TwoPhaseFileHeader/GlobalTransactionData

Hi,

In the undo system, we use full-transaction-id for transactions. For
rollback of prepared transactions, we were planning to use
FullTransactionId by combining TransactionId and epoch, but as
suggested by multiple people in that email chain [1][2], the better
idea is to store Full-transactionid in TwoPhaseFileHeader

Backward compatibility need not be handled for this scnario as upgrade
does not support having open prepared transactions.

There is also one more comment which is yet to be concluded. The
comment discusses about changing subxids which are of TransactionId
type to FullTransactionId type being written in two phase transaction
file. We could not conclude this as the data is similarly stored in
TransactionStateData.

Please find the patch having the fix for Storing FullTransactionId in
TwoPhaseFileHeader/GlobalTransactionData.

Let me know your opinion on the patch and above comment.

[1]
https://www.postgresql.org/message-id/CA%2BhUKGJ%2BPg2gE9Hdt6fXHn6ezV7xJnS%2Brm-38ksXZGXYcZh3Gg%40mail.gmail.com
[2]
https://www.postgresql.org/message-id/CAA4eK1L9BhvnQfa_RJCTpKQf9QZ15pyUW7s32BH78iBC3KbV0g%40mail.gmail.com

Regards,
Vignesh
EnterpriseDB: http://www.enterprisedb.com