Use constant macro for config cache 'magic trailer'

Change-Id: I76c5d25f95006b61bd96dbe7f6de757506e1c6e6
Reviewed-on: http://review.couchbase.org/29456
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>