Fix a startup issue where the QT Wallet is getting hung
Fix a startup issue where the QT Wallet is getting hung

Seems at times, especially after an interrupted node sync, that
the wallet will appear to get hung while the startup thread completes
ActivateBestChain(). This probably has something to do with thread
priorities on Windows however it's not entirely clear. So a simple hack
is to wait 500 ms at the beginning of ThreadImport() to allow the main
thread to return and redraw the QT wallet before proceding with
ActivateBestChain().

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