mon/OSDMonitor: limit MOSDMap message size by bytes

mon/OSDMonitor: limit MOSDMap message size by bytes

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit dcf1cd00cc6e2595c283b4a73a4de9df50e7330a)

Conflicts:
	src/mon/OSDMonitor.cc:
		1. replace g_conf() with g_conf, g_conf was replaced
with g_conf() to prepare for the crimson project. and that change was
not backported to luminous. so restore all `g_conf()` to `g_conf` in
this backport.
		2. replace std::min() and std::max() with MIN() and
MAX() macros, as we modernize it in an unrelated change. and that
change was not backported to luminous. so let's be conservative, and
restore to the old style macros.