Support pgbouncer over 1.8.0

[madboxkr]

SHOW STATS's results is changed after 1.8.x

/usr/local/lib/python2.7/dist-packages/newrelic_plugin_agent/plugins# diff -u0 pgbouncer.py pgbouncer.py.v1.8.1
--- pgbouncer.py	2018-03-06 13:12:09.462992560 +0900
+++ pgbouncer.py.v1.8.1	2018-03-06 13:11:02.278094643 +0900
@@ -41 +41 @@
-                                  database['total_requests'])
+                                  database['total_query_count'])
@@ -46 +46 @@
-            requests += database['total_requests']
+            requests += database['total_query_count']