Add NODE_BITCOIN_CASH service bit

Summary:
This is a port of Unlimited's PR705 which introduces a service bit for Bitcoin
Cash nodes. This helps differentiate nodes which support the fork ahead of
fork time, which is valuable for DNS seeders.

Test Plan:
make check
../qa/pull-tester/rpc-tests.py -extended

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D366