Only update the updateSmartFeeLabel once in sync

Summary:

Calling updateSmartFeeLabel and therefore estimateSmartFee is pointless during IBD.

GUI freezes appear because estimateSmartFee competes with processBlock for the m_cs_fee_estimator lock leading to multiple seconds of blocking the GUI thread in updateSmartFeeLabel.

This is a backport of core-gui#97

Test Plan: ninja && src/qt/bitcoin-qt

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10516