Out of scope LSNs include pre initdb LSNs, and LSNs prior to
latest_gc_cutoff.
To get there there was also two cleanups:
* Fix error handling in Execute message handler. This fixes behaviour
when basebackup retured an error. Previously pageserver thread just
died.
* Remove "ancestor" file which previously contained ancestor id and
branch lsn. Currently the same data can be obtained from metadata file.
And just the way we handled ancestor file in the code introduced the
case when branching fails timeline directory is created but there is no data in it
except ancestor file. And this confused gc because it scans
directories. So it is better to just remove ancestor file and clean up
this timeline directory creation so it happens after all validity
checks have passed
This introduces new timeline field latest_gc_cutoff. It is updated
before each gc iteration. New check is added to branch_timelines to
prevent branch creation with start point less than latest_gc_cutoff.
Also this adds a check to get_page_at_lsn which asserts that lsn at
which the page is requested was not garbage collected. This check
currently is triggered for readonly nodes which are pinned to specific
lsn and because they are not tracked in pageserver garbage collection
can remove data that still might be referenced. This is a bug and will
be fixed separately.
