Currently, if a host is unreachable when gpstart is run, it will not report this
and will instead fail with an error that is both inaccurate and unhelpful to the
user, such as claiming that checksums are invalid for segments on a given host
when it simply can't reach that host to verify the checksums.

This commit adds a check to verify that all hosts are reachable before beginning
the startup process and, if one or more hosts are not reachable, marks segments
on those hosts down (in gparray, not in the cluster) so gpstart won't try to run
any checks against unreachable hosts and so that the cluster can still be started
in this state so long as there are otherwise enough valid segments to start it.