netsync: don't update mempool/fee estimator unless we're synced up

[kcalvinalvin]

Noticed during ibd that there's a slight overhead for handleBlockchainNotification on mempool/fee estimator updates. Since there's no reason to be looking at this while we're not caught up, return early and avoid the calls.

[coveralls]

Pull Request Test Coverage Report for Build 7606526642

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 56.785%

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Totals
Change from base Build 7546934558:	0.005%
Covered Lines:	29211
Relevant Lines:	51441

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💛 - Coveralls