MB-9730 Add support to cancel running viewgroup cleanup process

Implemented an stdin listener thread for receiving "exit"
message and die current viewgroup cleanup process immediately
without completion.

Change-Id: I2ff9305ac15fb02f9273a745439558f2846e552b
Reviewed-on: http://review.couchbase.org/30990
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>