road to shielded activity

[panleone]

This is basically the last important sapling feature missing in MPW right now, shielded txs are poorly handled by the activity widget.
A list of tasks that needs to be done to finish the implementation:

• Network refactor. Done in Network refactor #323
• Unify transparent and shield sync. Done in Unify shield and transparent sync #340
• Keep track of owned nullifiers, in order to quickly decrypt a sapling spend. Done in pivx-shield#81
• When handling a block, make the shield library return transactions that belong to the wallet. Done in pivx-shield#81
• Put everything together and implement the activity. Review needed at Shield Activity #416