On-demand WAL download in create logical replication slot

[ololobus]

We have 3 open PRs to all Postgres versions

• Add on-demand WAL download in CreateReplicationSlot (v14) postgres#480
• Add on-demand WAL download in CreateReplicationSlot (v15) postgres#470
• Add on-demand WAL download in CreateReplicationSlot (v16) postgres#469
• Also do not forget to add the same change to PG17 version?

All are approved, so let's properly merge and bump submodule versions

Heikki also mentioned the tests' absence (can we write some?) and other discrepancies https://neondb.slack.com/archives/C04DGM6SMTM/p1725445174109119?thread_ts=1724867700.274789&cid=C04DGM6SMTM

[knizhnik]

There 5 where now we install our on-demand download functions:
3 times this set of functions is replaced:

       xlr.page_read = read_local_xlog_page;
	xlr.segment_open = wal_segment_open;
	xlr.segment_close = wal_segment_close;

and two times this set of function is replaced:

xlr.page_read = logical_read_xlog_page;
xlr.segment_open = WalSndSegmentOpen;
xlr.segment_close = wal_segment_close;

Where are using three different callbacks for it: SlotFuncs_Custom_XLogReaderRoutines, WalSender_Custom_XLogReaderRoutine and LogicalFuncs_Custom_XLogReaderRoutines all of which are not install the same set of functions:

	xlr->page_read = NeonWALPageRead;
	xlr->segment_open = NeonWALReadSegmentOpen;
	xlr->segment_close = NeonWALReadSegmentClose;

All this contexts are passed to CreateInitDecodingContext and CreateDecodingContext which in turn call StartupDecodingContext.

I wonder if we should define just one callback and call it from StartupDecodingContext.

[ololobus]

Looks like all PRs have been merged, slosing