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>
CCBC-205: Stricter/More inspectable behavior for config cache

This provides a test and an additional lcb_cntl operation to check the
status of the configuration cache. Currently this is a boolean value. In
the case where a more complex status may be desired, a new lcb_cntl
command may be added.

Change-Id: I4d40d29363ee530438c9309aef9cdcbf99589d10
Reviewed-on: http://review.couchbase.org/29457
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>