improve wallet's scan_input API

Today I intent to override wallet's scan_input method, because I do not want to adjust the balance if input's out_ref is claim_ticket_output,

But I found that input's output, my->_output_ref_to_index is private, so in's output cannot be accessed from derived class lotto_wallet with current API.

This pull request is to improve the scan_input api to keep similar approach with scan_output, added extra parameter(const output_index& idx), then claim_func can be identified.

https://github.com/HackFisher/bitshares_toolkit/blob/lotto_experi/libraries/lotto/lotto_wallet.cpp