src/: remove all direct comparisons to get_max()

src/: remove all direct comparisons to get_max()

get_max() now returns a special singleton type from which hobject_t's
can be assigned and constructed, but which cannot be directly compared.

This patch also cleans up all such uses to use is_max() instead.

This should prevent some issues like 16113 by preventing us from
checking for max-ness by comparing against a sentinel value.  The more
complete fix will be to make all fields of hobject_t private and enforce
a canonical max() representation that way.  That patch will be hard to
backport, however, so we'll settle for this for now.

Fixes: http://tracker.ceph.com/issues/16113
Signed-off-by: Samuel Just <sjust@redhat.com>
(cherry picked from commit 7c5f2acaa57bd6caaf4b13b48154df3ad6fbe84d)