## Problem
Previously, Timeline::gc_info was only updated in a batch operation at
the start of GC. That means that timelines didn't generally have
accurate information about who their children were before the first GC,
or between GC cycles.
Knowledge of child branches is important for calculating layer
visibility in #8398
## Summary of changes
- Split out part of refresh_gc_info into initialize_gc_info, which is
now called early in startup
- Include TimelineId in retain_lsns so that we can later add/remove the
LSNs for particular children
- When timelines are added/removed, update their parent's retain_lsns
2ef8e57f86