Rename Messagestart into DiskMagic and CashMessageStart into NetMagic
Summary: It make things clearer.
Test Plan:
make check ../qa/pull-tester/rpc-tests.py
Reviewers: schancel, freetrader, #bitcoin_abc
Reviewed By: schancel, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D666