crimson/{mgr,osd}: add WithStats::update_stats()

crimson/{mgr,osd}: add WithStats::update_stats()

because we need to implement a tell command which forces osd to
send the latest pg stats to mgr, and the command returns the user with
the sequence id of the report, and `mgr::Client::report()` does not
return a future, so we have to update the seq id before sending
the report. the solution is to update the seq id in a separated
method, so in this change:

* add `const` to `WithStats::get_stats()
* add a dedicated method of `WithStats::update_stats()` to update
  the stats to be collected by mgr::Client.

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