Move BitcoinApplication to header so it can be tested

Summary:
Move-only commit, no code changes

Partial backport of core PR11625 (commit 7e4bd19)
https://github.com/bitcoin/bitcoin/pull/11625/commits/ca20b65cc04825bb317f1a59d02c77912f6bf097

Test Plan:

make check
ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4497