Move to clang format 8

Summary: clang-format 8 is available on all systems we support and version 7 is not packaged for brew.

Test Plan:

arc lint --everything

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D4539