Make functions in rpc/blockchain.cpp static.

Summary:
Partial backport of Core PR 13163
https://github.com/bitcoin/bitcoin/pull/13163/files#diff-a0c8f511d90e83aa9b5857e819ced344
Only partially completed since the original PR is massive.
Progress towards T644

Depends on D2951, D2946

Test Plan: make check

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2952