Sign with SIGHASH_FORKID when UAHF is activated.

Summary:
The wallet code will sign transactions in a replay protected manner by default to ensure user funds are safe once the fork activated.

Depends on D312, D310

Test Plan:
Many integration test are testing the signing code of the wallet, they are all still green.
I'd like to run live test on testnet soon, but we need to land that code first.

Reviewers: #bitcoin_abc, freetrader

Reviewed By: #bitcoin_abc, freetrader

Subscribers: freetrader

Differential Revision: https://reviews.bitcoinabc.org/D327