High-level ideas:
- placeholder Timeline object in timelines map during a timeline creation
- the timeline creations (branch, bootstrap, import_from_basebackup)
prepare durable state (on-disk & remote)state, if necessary using
_another_ _temporary_ Timeline object
- once the timeline creations have prepared the durable state, they
use the normal load routine (load_local_timeline) that is also used
during pageserver startup
- Once the loading is done, we replace the placheolder timeline object
with the real one