wire, netsync: change isSyncCandidate behavior

[kcalvinalvin]

isSyncCandidate is now changed to return true even if the peer is a pruned node if and only if our chaintip is within 288 blocks of the peer.

Rationale being that pruned nodes that signal NODE_NETWORK_LIMITED MUST serve 288 blocks from their chaintip. If our chaintip is within that range, this peer can be a sync candidate even if they aren't an archival node.

[Roasbeef]

cc @ellemouton

[coveralls]

Pull Request Test Coverage Report for Build 6779126378

• 3 of 40 (7.5%) changed or added relevant lines in 2 files are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.02%) to 55.689%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
netsync/manager.go	0	37	0.0%
<!--	Total:	3	40	7.5%	-->

Files with Coverage Reduction	New Missed Lines	%
mempool/mempool.go	1	66.75%
peer/peer.go	6	74.16%
<!--	Total:	7		-->

Totals
Change from base Build 6776433285:	-0.02%
Covered Lines:	27150
Relevant Lines:	48753

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💛 - Coveralls