Fix powa_stat_archiver_src for pg 10-

And also run ci for pg 9.6 to test this code path.

.github/workflows/tests.yml

@@ -20,6 +20,7 @@ jobs:
     strategy:
       matrix:
         postgres_major_version: [
+          "9.6",
           "10",
           "11",
           "12",

powa--5.0.0dev.sql

@@ -4448,7 +4448,7 @@ BEGIN
             v_current_wal := NULL;
         ELSE
             IF current_setting('server_version_num')::int < 100000 THEN
-                v_current_wal :=  pg_walfile_name(pg_last_wal_receive_lsn());
+                v_current_wal :=  pg_xlogfile_name(pg_current_xlog_location());
             ELSE
                 v_current_wal :=  pg_walfile_name(pg_current_wal_lsn());
             END IF;