Increase wire.MaxBlockPayload to 8MB

[roylee17]

This variable seems redundant to the blockchain.MaxBlockBaseSize to make the wire and blockchain self-contained. We've updated the blockchain.MaxBlockBaseSize to 8MB for LBRY, but missed this one?

[coveralls]

Pull Request Test Coverage Report for Build 1804753474

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 51.822%

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Files with Coverage Reduction	New Missed Lines	%
peer/peer.go	2	76.16%
<!--	Total:	2		-->

Totals
Change from base Build 1782808406:	0.02%
Covered Lines:	22327
Relevant Lines:	43084

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💛 - Coveralls

[BrannonKing]

This PR has the fee functions as well; that doesn't seem quite right. As for the maximum block size, your change looks correct: https://github.com/lbryio/lbrycrd/blob/master/src/consensus/consensus.h