crimson/osd: create msgrs in main.cc

crimson/osd: create msgrs in main.cc

messengers are sharded<Service>. we should not create them in another
sharded service's start() method. to ensure the ordering of stop of
sharded services, we should create the sharded services in main().
and register their stop() method in the proper order.

Signed-off-by: Kefu Chai <kchai@redhat.com>