Share code for srv_session::close() so AC is correctly updated

Summary: Connections through the srv_session API currently do not decrement AC connection counts since srv_session::close() largely duplicates the contents of end_connection. Rather than copy over the multitenancy call, it seems like it would be smarter to have them share common functionality so if anything else gets added it doesn't need to be added to multiple locations

Reviewed By: george-reynya

Differential Revision: D40443752

fbshipit-source-id: 615ece1

Author: Aaron Balsara

mysql-test/suite/test_service_sql_api/r/test_session_audit_null.result

@@ -51,7 +51,7 @@ SHOW STATUS LIKE 'audit_null_connection%';
 Variable_name	Value
 Audit_null_connection_change_user	0
 Audit_null_connection_connect	0
-Audit_null_connection_disconnect	0
+Audit_null_connection_disconnect	10
 Audit_null_connection_pre_authenticate	0
 ##########################################
 # Stop audit_null plugin