common/config: extract ObserverMgr from md_config_impl<>

common/config: extract ObserverMgr from md_config_impl<>

crimson will have sharded<PGShard> (name may vary in future), and
underlying ObjectStore instances. they will be consumers of crimson
ConfigProxy, and will register them as an observer of config changes.
but as always, in crimson osd, world is sharded, each ConfigProxy will
only take care of the sharded services living on the shard which it is
in charge of. in other words, unlike alien's ConfigProxy, the
"observers" data structure is not shared, they are sharded as well. so
we need to extract the observers out of md_config_impl<>, and move it
into ConfigProxy.

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