Revert "source-postgres-batch: Fix XID wraparound behavior"

This reverts commit e57acbc
and un-fixes #2383

Turns out the particular function we're using here doesn't
work on a standby replica, which is...bad.

source-postgres-batch/.snapshots/TestQueryTemplates-XMinFirstQuery

@@ -1,4 +1 @@
-SELECT xmin AS txid, * FROM "test"."foobar"
-    WHERE xmin::text::bigint >= 3
-      AND (((txid_current() - xmin::text::bigint)<<32)>>32) >= 0
-    ORDER BY xmin::text::bigint;
+SELECT xmin AS txid, * FROM "test"."foobar" ORDER BY xmin::text::bigint;

source-postgres-batch/.snapshots/TestQueryTemplates-XMinSubsequentQuery

@@ -1,5 +1,3 @@
 SELECT xmin AS txid, * FROM "test"."foobar"
-    WHERE xmin::text::bigint >= 3
-      AND (((txid_current() - xmin::text::bigint)<<32)>>32) >= 0
-      AND (((xmin::text::bigint - $1::bigint)<<32)>>32) > 0
+    WHERE (((xmin::text::bigint - $1::bigint)<<32)>>32) > 0 AND xmin::text::bigint >= 3
     ORDER BY (((xmin::text::bigint - $1::bigint)<<32)>>32);

source-postgres-batch/main.go

@@ -168,15 +168,10 @@ func selectQueryTemplate(res *Resource) (string, error) {
 // the XID epoch wraps around. If this assumption is violated it would in principle be doable
 // to `SELECT txid_current() as polled_txid, ...` and use "polled_txid" as the cursor value.
 const tableQueryTemplateXMIN = `{{if .IsFirstQuery -}}
-  SELECT xmin AS txid, * FROM {{quoteTableName .SchemaName .TableName}}
-    WHERE xmin::text::bigint >= 3
-      AND (((txid_current() - xmin::text::bigint)<<32)>>32) >= 0
-    ORDER BY xmin::text::bigint;
+  SELECT xmin AS txid, * FROM {{quoteTableName .SchemaName .TableName}} ORDER BY xmin::text::bigint;
 {{- else -}}
   SELECT xmin AS txid, * FROM {{quoteTableName .SchemaName .TableName}}
-    WHERE xmin::text::bigint >= 3
-      AND (((txid_current() - xmin::text::bigint)<<32)>>32) >= 0
-      AND (((xmin::text::bigint - $1::bigint)<<32)>>32) > 0
+    WHERE (((xmin::text::bigint - $1::bigint)<<32)>>32) > 0 AND xmin::text::bigint >= 3
     ORDER BY (((xmin::text::bigint - $1::bigint)<<32)>>32);
 {{- end}}`