Various nits in the RCU mechanism

Summary:
Remove uneeded include.
Rename variable with poor name foo.
Remove redundant loads as compare_exchange_weak update expected value when the update fails.
Fix copyright notice.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2300