Overwrite the orphan list periodically instead of using heuristics

[and-cb]

The orphan page list has some complicated logic around it so that it is "crash recoverable" to some extent. This logic is showing some flaws that are described in #151 and are reproduced by the new test in this PR.

Rather than trying to fix the logic, this PR changes it completely: pushing new items to the orphan page list now always appends items to the end of it. At commit time, if there's enough room, the orphans are moved to the start of the list, overwriting the reclaimed orphans.

This makes commits more expensive, but guarantees that all orphans are eventually used (as long as snapshots are non-decreasing), and also the amortized cost of adding orphans is still O(1).

TODO

• Update comments and documentation
• Maybe add a check to push() to ensure that snapshots are non-decreasing
• Rename the test push_pop_2 to something better
• Remove the debug code in pop()

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2

[BrianBland]