MEDIUM: Split up struct server's check element

MEDIUM: Split up struct server's check element

This is in preparation for associating a agent check
with a server which runs as well as the server's existing check.

The split has been made by:
* Moving elements of struct server's check element that will
  be shared by both checks into a new check_common element
  of struct server.
* Moving the remaining elements to a new struct check and
  making struct server's check element a struct check.
* Adding a server element to struct check, a back-pointer
  to the server element it is a member of.
  - At this time the server could be obtained using
    container_of, however, this will not be so easy
    once a second struct check element is added to struct server
    to accommodate an agent health check.

Signed-off-by: Simon Horman <horms@verge.net.au>