Fix powa_stat_archiver_src for pg 10-

And also run ci for pg 9.6 to test this code path.  Surprisingly the ci fails
for pg9.6 for any attemp immeditely after a successful pg_ctl start, so I'm
adding a 1 second sleep, which seems to be reliable enough.

.github/workflows/tests.yml

@@ -20,6 +20,7 @@ jobs:
     strategy:
       matrix:
         postgres_major_version: [
+          "9.6",
           "10",
           "11",
           "12",
@@ -79,6 +80,8 @@ jobs:
         pg_ctl -D $DATADIR initdb
         echo "shared_preload_libraries = 'pg_stat_statements'" >> $DATADIR/postgresql.conf
         pg_ctl -D $DATADIR -l $LOGFILE start || cat $LOGFILE
+        # a sleep is required for pg9.6 (at least)
+        sleep 1
         psql -c 'select 1 as ok' postgres
 
     - name: Build and install powa-archivist for postgres ${{ matrix.postgres_major_version }}

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;