addrmgr: make KnownAddress methods thread-safe

[chappjc]

This is an alternative resolution for the data races in #1760. I'm proposing both solutions in case one is more palatable to the maintainers than the other. Note that the approach in this PR was taken by dcrd's fix in decred/dcrd@ad33b0e

This gives KnownAddress a sync.RWMutex so the exported methods may safely access the na (*wire.NetAddress) and lastattempt fields.
The AddrManager is updated to lock the new KnownAddress mutex before assigning to na or lastattempt.
The other KnownAddress fields are only accessed by AddrManager, using its own mutex for synchronization.

This resolution requires no code changes in consumers like neutrino, just an update to the btcd module requirement. I've tested this with btcwallet --testnet --usespv -d debug, which uses neutrino and the affected code, using just a replace github.com/btcsuite/btcd => github.com/chappjc/btcd v0.22.0-beta.0.20211020165918-67e82a6dbe7f.

[jcvernaleo]

Thanks! I'll try to review this tomorrow but without looking at any of the code, my guess is that it makes the most sense to follow what decred does when we can (the fork becomes the upstream :) )

[coveralls]

Pull Request Test Coverage Report for Build 1364556215

• 15 of 20 (75.0%) changed or added relevant lines in 2 files are covered.
• 6 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.01%) to 52.98%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
addrmgr/knownaddress.go	4	6	66.67%
addrmgr/addrmanager.go	11	14	78.57%

Files with Coverage Reduction	New Missed Lines	%
addrmgr/addrmanager.go	1	73.51%
integration/rpctest/blockgen.go	2	83.67%
btcec/signature.go	3	92.82%

Totals
Change from base Build 1308902808:	0.01%
Covered Lines:	21100
Relevant Lines:	39826

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💛 - Coveralls

[chappjc]

Thanks! I'll try to review this tomorrow but without looking at any of the code, my guess is that it makes the most sense to follow what decred does when we can (the fork becomes the upstream :) )

Yep that makes sense. No point in diverging unnecessarily.

[jcvernaleo]

Looks all good to me.
OK