fix(layer): remove the need to repair internal state (#7030)

## Problem

The current implementation of struct Layer supports canceled read
requests, but those will leave the internal state such that a following
`Layer::keep_resident` call will need to repair the state. In
pathological cases seen during generation numbers resetting in staging
or with too many in-progress on-demand downloads, this repair activity
will need to wait for the download to complete, which stalls disk
usage-based eviction. Similar stalls have been observed in staging near
disk-full situations, where downloads failed because the disk was full.

Fixes #6028 or the "layer is present on filesystem but not evictable"
problems by:
1. not canceling pending evictions by a canceled
`LayerInner::get_or_maybe_download`
2. completing post-download initialization of the `LayerInner::inner`
from the download task

Not canceling evictions above case (1) and always initializing (2) lead
to plain `LayerInner::inner` always having the up-to-date information,
which leads to the old `Layer::keep_resident` never having to wait for
downloads to complete. Finally, the `Layer::keep_resident` is replaced
with `Layer::is_likely_resident`. These fix #7145.

## Summary of changes

- add a new test showing that a canceled get_or_maybe_download should
not cancel the eviction
- switch to using a `watch` internally rather than a `broadcast` to
avoid hanging eviction while a download is ongoing
- doc changes for new semantics and cleanup
- fix `Layer::keep_resident` to use just `self.0.inner.get()` as truth
as `Layer::is_likely_resident`
- remove `LayerInner::wanted_evicted` boolean as no longer needed

Builds upon: #7185. Cc: #5331.

pageserver/src/tenant/mgr.rs

@@ -2,7 +2,6 @@
 //! page server.
 
 use camino::{Utf8DirEntry, Utf8Path, Utf8PathBuf};
-use futures::stream::StreamExt;
 use itertools::Itertools;
 use pageserver_api::key::Key;
 use pageserver_api::models::ShardParameters;
@@ -1662,9 +1661,9 @@ impl TenantManager {
                     .layers
                     .read()
                     .await
-                    .resident_layers()
-                    .collect::<Vec<_>>()
-                    .await;
+                    .likely_resident_layers()
+                    .collect::<Vec<_>>();
+
                 for layer in timeline_layers {
                     let relative_path = layer
                         .local_path()