wire: add method TxID to MsgTx

[ffranr]

This commit adds a method to MsgTx called TxID. This method returns the transaction ID (txid) of the subject transaction. This commit also adds a function for deriving the transaction ID from the double hash of the transaction.

[coveralls]

Pull Request Test Coverage Report for Build 8419979356

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 56.862%

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Totals
Change from base Build 8334044258:	0.006%
Covered Lines:	29421
Relevant Lines:	51741

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💛 - Coveralls

[ffranr]

I am aware of chainhash.Hash.String(), but I still think this PR is a good idea.

This PR makes it clear what txid is in relation to MsgTx. It isn't clear that chainhash.Hash.String() is txid. And it isn't necessarily always txid because chainhash.Hash doesn't have to be a double sha256.

[kcalvinalvin]

I think it's ok to have a method that returns a string directly.

But there's no need for all this code to do so.

[saubyk]

@yyforyongyu for your review