Fix numerous declaratiosn shadowing others

Summary: Eventually we'll get to a point where we can enable -Wshadow by default.

Test Plan:
Add -Wshadow as a compler flag

make

Ensure numerous warnings are gone.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1865