[avalanche] Move proof verification to processor

Summary:
This is an attempt to fix the processor initialization by adding a
factory method to build a Processor which will check the input arguments
consistency and build a Processor instance accordingly, or return a null
ptr and an error string. The Processor is only built if the arguments
are right. A few additional checks are added for the private keys.

This is based on D9368.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9440