refactor timeline initialization

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

libs/pageserver_api/src/models.rs

@@ -126,6 +126,7 @@ impl std::fmt::Debug for TenantState {
 /// A state of a timeline in pageserver's memory.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
 pub enum TimelineState {
+    Creating,
     /// The timeline is recognized by the pageserver but is not yet operational.
     /// In particular, the walreceiver connection loop is not running for this timeline.
     /// It will eventually transition to state Active or Broken.