[PRE-MERGE] hlinnaka/walingest-simplify-vm-flag-clearing-2

Squashed commit of the following:

commit b1d701dc06
Author: Heikki Linnakangas <heikki@neon.tech>
Date:   Thu Jan 4 18:23:59 2024 +0200

    Refactor generation of ClearVisibilityMapFlags records.

    With fewer mutable variables, for sake of clarity.

commit aa95a07d27
Author: Heikki Linnakangas <heikki@neon.tech>
Date:   Thu Jan 4 18:20:03 2024 +0200

    Refactor code to apply ClearVisibilityMapFlags records a little.

    To reduce the repetition.

commit 18e9208158
Author: John Spray <john@neon.tech>
Date:   Thu Jan 4 10:40:03 2024 +0000

    pageserver: improved error handling for shard routing error, timeline not found (#6262)

    ## Problem

    - When a client requests a key that isn't found in any shard on the node
    (edge case that only happens if a compute's config is out of date), we
    should prompt them to reconnect (as this includes a backoff), since they
    will not be able to complete the request until they eventually get a
    correct pageserver connection string.
    - QueryError::Other is used excessively: this contains a type-ambiguous
    anyhow::Error and is logged very verbosely (including backtrace).

    ## Summary of changes

    - Introduce PageStreamError to replace use of anyhow::Error in request
    handlers for getpage, etc.
    - Introduce Reconnect and NotFound variants to QueryError
    - Map the "shard routing error" case to PageStreamError::Reconnect ->
    QueryError::Reconnect
    - Update type conversions for LSN timeouts and tenant/timeline not found
    errors to use PageStreamError::NotFound->QueryError::NotFound

libs/utils/src/http/error.rs

@@ -31,6 +31,9 @@ pub enum ApiError {
     #[error("Shutting down")]
     ShuttingDown,
 
+    #[error("Timeout")]
+    Timeout(Cow<'static, str>),
+
     #[error(transparent)]
     InternalServerError(anyhow::Error),
 }
@@ -67,6 +70,10 @@ impl ApiError {
                 err.to_string(),
                 StatusCode::SERVICE_UNAVAILABLE,
             ),
+            ApiError::Timeout(err) => HttpErrorBody::response_from_msg_and_status(
+                err.to_string(),
+                StatusCode::REQUEST_TIMEOUT,
+            ),
             ApiError::InternalServerError(err) => HttpErrorBody::response_from_msg_and_status(
                 err.to_string(),
                 StatusCode::INTERNAL_SERVER_ERROR,