osd/scrub: make m_session_started_at at Session state ctor

osd/scrub: make m_session_started_at at Session state ctor

ScrubMachine::get_time_scrubbing() must access the Session object
to compute the scrub duration. But the State data is not externally
accessible before its ctor has completed.

As we always happen to try to access that data inside the ctor,
this always results in a warning log message.

Here we move m_session_started_at into the outer state, simplifying
the logic required to access it.

Fixes: https://tracker.ceph.com/issues/64955
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>