mon: add a STRETCH_MODE feature and check it

mon: add a STRETCH_MODE feature and check it

We look at the up OSDs when enabling stretch mode to make sure the running
ones can handle it. OSDs require their peers to support stretch mode
when connecting, via OSDMap::get_features(). Monitors reject booting OSDs
if stretch mode is enabled and the OSD doesn't support it.

The monitors are already validated and gated by having CONNECTIVITY
election mode enabled.

Signed-off-by: Greg Farnum <gfarnum@redhat.com>