osdc: Update to use C++11 concurrency

osdc: Update to use C++11 concurrency

The only externally visible change is that Objecter::get_osdmap_read
and Objedcter::put_osdmap_read have been removed in favor of
Objecter::with_osdmap.

It can be used like:

objecter->with_osdmap([&](const OSDMap& o) { o.do_stuff(); };
int x = objecter->with_osdmap([&](const OSDMap& o) {
    return get_thing(id); });
objecter->with_osdmap(std::mem_fn(&OSDMap::do_stuff));
int x = objecter->with_osdmap(std::mem_fn(&OSDMap::get_thing),
                              id);

The choice between the style of the first two examples or the second two
is arbitrary and depends on what you prefer.

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>