Make system ID generation safe by modifying algorithm

TODO: this algorithm will fail if someone deletes a user that is not the last one.
File: /api/src/main/java/org/openmrs/api/db/hibernate/HibernateUserDAO.java
Line: 390
Patch:

Index: api/src/main/java/org/openmrs/api/db/hibernate/HibernateUserDAO.java
===================================================================
— api/src/main/java/org/openmrs/api/db/hibernate/HibernateUserDAO.java (revision 26806)
+++ api/src/main/java/org/openmrs/api/db/hibernate/HibernateUserDAO.java (working copy)
@@ -388,6 +388,7 @@
public Integer generateSystemId() {

// TODO this algorithm will fail if someone deletes a user that is not the last one.
+ // Issue was added: https://tickets.openmrs.org/browse/TRUNK-3252

String sql = "select count(user_id) as user_id from users";