Add support for receiving sendaddrv2 message from a peer

[yaslama]

Fixes #1661

[shahriyar9839]

0x915133E981cD5D9616F191977a8896522d56Cd7C

[Roasbeef]

Two small comment nits, but other than that should resolve the current issue w.r.t neutrino peers being able to connect to bitcoind master, and their upcoming release as well. I still wonder why a protocol version bump wasn't used instead of introducing a new message, but hey we're already here 🤷‍♂️

[coveralls]

Pull Request Test Coverage Report for Build 395832549

• 0 of 17 (0.0%) changed or added relevant lines in 2 files are covered.
• 12 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.05%) to 53.369%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
wire/message.go	0	2	0.0%
wire/msgsendaddrv2.go	0	15	0.0%

Files with Coverage Reduction	New Missed Lines	%
peer/peer.go	2	75.47%
btcec/signature.go	3	92.48%
connmgr/connmanager.go	3	86.07%
database/ffldb/blockio.go	4	92.62%

Totals
Change from base Build 381748156:	-0.05%
Covered Lines:	20824
Relevant Lines:	39019

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💛 - Coveralls

[jakesylvestre]

Looks good to me

[Roasbeef]

LGTM 🥟

Will update the neutrino project as well so the main package in isolation (when not used with lnd which will also pull in this change) can speak with the latest bitcoind nodes.