Add a MailService sharedSession option

To avoid having a single mail session instance shared across all lookups, we need the option to create a new mail session on each jndi lookup. A new sharedSession attribute is needed to control this behavior:

/**

• Set whether a single mail session should be shared across all lookups
• (sharedSession = true) or a new session created on each lookup
• (sharedSession = false, the default).
• @param sharedSession
  */
  public void setSharedSession(boolean sharedSession)