rpc abortrescan

bitcoin core has this call available. I think this will be useful especially since there are some cases where rescan can fail, but the blockchain continues to feed wallet rescan blocks (resulting in "error rescan block too high") until the ENTIRE chain has played out.

```
Stops current wallet rescan triggered by an RPC call, e.g. by an importprivkey call.
Note: Use "getwalletinfo" to query the scanning progress.

Result:
true|false (boolean) Whether the abort was successful

Examples:

Import a private key
> bitcoin-cli importprivkey "mykey"

Abort the running wallet rescan
> bitcoin-cli abortrescan
```