Make CNodeStats a struct and add a blurb explaining what it is.

Summary: As per title. Hopefully this will make it clearer how that whole thing work.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2126