Activate persistent subscriptions when they are recreated.

Prior to this change, recreating persistent subscriptions would
create the subscription but would not activate it. This led to subscriptions
being listed in the "NULL" state by diagnostics and not sending NOTIFYs
when expected.

Review: https://reviewboard.asterisk.org/r/4261



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@429571 f38db490-d61c-443f-a65b-d21fe96a405b