Introduce new PageTracer functionalities

[vidyasilai]

• This PR refactors the trace_refs_recursive function to use a new interface called PageTracer. This is the first step in a solution to the issue described in Eliminate redundant page loads when tracing outgoing references  #170, with the new functionality being encapsulated in the PageTracer class.
• Subsequent PRs will implement the actual logic to avoid the redundant page load.

[tonyastolfi]

Thanks for your PR... lots to like in this one wrt completeness and good style!

I would like to resolve some high-level design concerns before moving ahead. Please read my comment on page_tracer.hpp and LMK what you think.

[tonyastolfi]

Looks great for the most part, just a few minor comments. I really appreciate the attention paid to designing and implementing a comprehensive test case!