app_voicemail: IMAP connection control

A new global option "imap_poll_logout" was added to specify whether need to
disconnect from the IMAP server after polling of mailboxes.

ASTERISK-27068 #close

Closing IMAP connection after loading mailbox from voicemail.conf

ASTERISK-24052 #close

Change-Id: Ib7558ba04516240a32b65f42e9be64372a0ae12a