NotificationService should not keep same DB session forever

Steps to reproduce:

• Set MySQL wait_timeout to a short value: SET @@GLOBAL.wait_timeout=10;
• Start SonarQube server
• Send a first notification (update an issue) => session is opened and attached to NotificationService thread
• Wait for 10 seconds until idle connection is closed by MySQL server
• Try to send another notification => NotificationService try to reuse same previous session but underlying connection is closed => error

  WARN  o.s.s.n.NotificationService  Unable to deliver notification Notification
  [...]
  Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
  
  The last packet successfully received from the server was 181 088 milliseconds ago.  The last packet sent successfully to the server was 0 milliseconds ago.
  

In real life it occurs when the NotificationManager didn't send email in a period exceeding MySQL wait_timeout.

There is an easy fix that is to always close session after each execution of the NotificationService. This way during next "wake-up" of the NotificationService a fresh new session will be created and a new connection requested from the pool.