Block BDM during scanRegisteredTxForWallet.

This bugfix resolves a reproducible coredump resulting from BDM timeouts, while scanning a wallet's transactions.  

Since scanRegisteredTxForWallet can take a long time, block the BDM, which would timeout.

While setting and clearing blocking mode, announce the previous mode, to restore it.

Also, do not call through to the cpp layer using the wrong argument types!  wait=True gets dynamically munged into a valid argument, even though it's not.  (The correct third argument is endBlk, and it defaults appropriately.)