[Wallet] Call SetBestChain before and after rescan #4800

This ensures to redo rescan, in case of an incomplete rescan.

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Why? If I start a rescan, bitcoin-qt finds the tx I was missing and then I quit it, I dont expect it to continue to rescan, I expect it to restart into normal mode.

The software does a rescan, when it believes the balance shown to user could be wrong, because some transactions may be missing. This means, if we crash while rescanning, the balance and transactions can be in a wrong state, thus we need to ensure a complete rescan.

Now, for expert-users, we could introduce a 'rescan=0' to force not to continue rescan. Or we could only continue, if it was a forced rescan, and not if it was a voluntary rescan.

After thinking, this basically only affects the import-functions importwallet etc. So there is the bug, when someone imports a wallet and then we crash while rescanning. So maybe the better solution is to only call SetBestChain in the import-functions.

Closing this. Initially I thought, we would not continue rescan, in case someone loads a wallet-backup and crashes while rescanning. But this is not the case. And for the import-functions someone is probably smart enough to redo, in case his machine dies while the operation.