app_queue: Add LoginTime field for member in a queue.

Add a time_t logintime to storage a time when a member is added into a
queue.

Also, includes show this time (in seconds) using a 'queue show' command
and the field LoginTime for response for AMI events.

ASTERISK-18069 #close

Change-Id: Ied6c3a300f78d78eebedeb3e16a1520fc3fff190