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

Test Plan: mt test_service_sql_api.test_sql_complex

Reviewers: mung, greynya, #mysql_gateway

Reviewed By: greynya

Subscribers: webscalesql-eng@fb.com

Differential Revision: https://phabricator.intern.facebook.com/D40443752

Tasks: T134350458

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