Continue my quest to make 002_blocks.pl pass reliably.

The latest buildfarm failures show that after the insert, we don't
actually wait long enough for WAL summarization to catch up, apparently
because the on disk state gets updated before the in-memory state, and
so by checking the on disk state to see whether we're caught up and then
the in-memory state to see where exactly how far we've progressed, we
can, if unlucky, derive an older value of summarized_lsn, messing up
the rest of the test.

Attempt to fix this by using pg_available_wal_summaries() everywhere in
the test and pg_get_wal_summarizer_state() nowhere.

Per buildfarm.

src/bin/pg_walsummary/t/002_blocks.pl

@@ -46,12 +46,11 @@ SELECT EXISTS (
 EOM
 ok($result, "WAL summarization caught up after insert");
 
-# Get a list of what summaries we now have.
-my $progress = $node1->safe_psql('postgres', <<EOM);
-SELECT summarized_tli, summarized_lsn FROM pg_get_wal_summarizer_state()
+# Find the highest LSN that is summarized on disk.
+my $summarized_lsn = $node1->safe_psql('postgres', <<EOM);
+SELECT MAX(end_lsn) AS summarized_lsn FROM pg_available_wal_summaries()
 EOM
-my ($summarized_tli, $summarized_lsn) = split(/\|/, $progress);
-note("after insert, summarized TLI $summarized_tli through $summarized_lsn");
+note("after insert, summarized through $summarized_lsn");
 note_wal_summary_dir("after insert", $node1);
 
 # Update a row in the first block of the table and trigger a checkpoint.
@@ -65,7 +64,7 @@ EOM
 $result = $node1->poll_query_until('postgres', <<EOM);
 SELECT EXISTS (
     SELECT * from pg_available_wal_summaries()
-    WHERE tli = $summarized_tli AND end_lsn > '$summarized_lsn'
+    WHERE end_lsn > '$summarized_lsn'
 )
 EOM
 ok($result, "got new WAL summary after update");
@@ -73,7 +72,7 @@ ok($result, "got new WAL summary after update");
 # Figure out the exact details for the new summary file.
 my $details = $node1->safe_psql('postgres', <<EOM);
 SELECT tli, start_lsn, end_lsn from pg_available_wal_summaries()
-	WHERE tli = $summarized_tli AND end_lsn > '$summarized_lsn'
+	WHERE end_lsn > '$summarized_lsn'
 EOM
 my @lines = split(/\n/, $details);
 is(0+@lines, 1, "got exactly one new WAL summary");