app_voicemail: Improve msg_id handling

app_voicemail will no longer issue error messages when it retrieves an msg_id
with a NULL value from realtime and will instead simply populate the msg_id
field with a newly generated msg_id. In addition, this patch changes the way
msg_ids are generated to eliminate certain causes of duplicate IDs appearing
within a single system. In addition, when messages are copied, they will now
receive a new msg_id.

(closes issue ASTERISK-20717)
Reported by: Alec Davis
Review: https://reviewboard.asterisk.org/r/2220/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@379460 f38db490-d61c-443f-a65b-d21fe96a405b