Fix for error in 'getbalance' when using watchonly addresses/scripts.

Also added a warning to 'importaddress'

This bug can result in too much watchonly balance being returned by the getbalance RPC call.
Do we need this extra warning in addition to the **This is a pre-release test build - use at your own risk - do not use for mining or merchant applications** that already comes in pre-release builds by default?