feat(rtc): maintain dirty and uploaded IndexPart (#7833)

RemoteTimelineClient maintains a copy of "next IndexPart" as a number of
fields which are like an IndexPart but this is not immediately obvious.
Instead of multiple fields, maintain a `dirty` ("next IndexPart") and
`clean` ("uploaded IndexPart") fields.

Additional cleanup:
- rename `IndexPart::disk_consistent_lsn` accessor
`duplicated_disk_consistent_lsn`
- no one except scrubber should be looking at it, even scrubber is a
stretch
- remove usage elsewhere (pagectl used by tests, metadata scan endpoint)
- serialize index part *before* the index upload operation
- avoid upload operation being retried because of serialization error
- serialization error is fatal anyway for timeline -- it can only make
transient local progress after that, at least the error is bubbled up
now
- gather exploded IndexPart fields into single actual
`UploadQueueInitialized::dirty` of which the uploaded snapshot is
serialized
- implement the long wished monotonicity check with the `clean`
IndexPart with an assertion which is not expected to fire

Continued work from #7860 towards next step of #6994.

pageserver/ctl/src/index_part.rs

@@ -26,7 +26,7 @@ pub(crate) async fn main(cmd: &IndexPartCmd) -> anyhow::Result<()> {
 
             let output = Output {
                 layer_metadata: &des.layer_metadata,
-                disk_consistent_lsn: des.get_disk_consistent_lsn(),
+                disk_consistent_lsn: des.metadata.disk_consistent_lsn(),
                 timeline_metadata: &des.metadata,
             };