Usability improvements for z_importkey

- Add height parameter to z_importkey to reduce rescan range
- Change semantics of rescan parameter, so users can explicitly force a rescan
  for existing keys.

Closes #2032

src/rpcclient.cpp

@@ -111,7 +111,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "z_sendmany", 3},
     { "z_getoperationstatus", 0},
     { "z_getoperationresult", 0},
-    { "z_importkey", 1 }
+    { "z_importkey", 2 },
 };
 
 class CRPCConvertTable