Handle relation DROPs in page server.

Add back code to parse transaction commit and abort records, and in
particular the list of dropped relations in them. Add 'put_unlink'
function to the Timeline trait and implementation. We had the code to
handle dropped relations in the GC code and elsewhere in ObjectRepository
already, but there was nothing to create the RelationSizeEntry::Unlink
tombstone entries until now. Also add a test to check that GC correctly
removes all page versions of a dropped relation.

Implements https://github.com/zenithdb/zenith/issues/232, except for the
"orphaned" rels.

Reviewed-by: Konstantin Knizhnik

test_runner/batch_others/test_gc.py

@@ -84,3 +84,14 @@ def test_gc(zenith_cli, pageserver, postgres, pg_bin):
                     assert row['dropped'] == 0
                     assert row['truncated'] == 0
                     assert row['deleted'] == 0
+
+                    #
+                    # Test DROP TABLE checks that relation data and metadata was deleted by GC from object storage
+                    #
+                    cur.execute("DROP TABLE foo")
+
+                    pscur.execute(f"do_gc {timeline} 0")
+                    row = pscur.fetchone()
+                    print("GC duration {elapsed} ms, relations: {n_relations}, dropped {dropped}, truncated: {truncated}, deleted: {deleted}".format_map(row))
+                    # Each relation fork is counted separately, hence 3.
+                    assert row['dropped'] == 3