Mark fall-through cases in our code clearly

Summary:
This should help to avoid unnecessary compiler warnings.
In practice it does not seem to work right despite -Wimplicit-fallthrough=3,
but it does not hurt to clearly mark the cases already.

Links:
https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
https://developers.redhat.com/blog/2017/03/10/wimplicit-fallthrough-in-gcc-7/

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, sickpig, kyuupichan, awemany

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D224