Various improvements to the scheduler

Summary:

• Use proper C++ function (void) is a C construct and we are not using C
• Use std::bind instead of boost::bind
• Relayout a few comments

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3167