msg/async/ProtocolV2: use shared_ptr to manage auth_meta

msg/async/ProtocolV2: use shared_ptr to manage auth_meta

When we reconnect a session, we need to move the new connection's auth_meta
over to the existing connection.  However, the existing connection may
have a thread that is unlocked and calling into an AuthClient or AuthServer
method making good use of the old auth_meta.

Resolved this by making auth_meta a shared_ptr and taking a local ref
before dropping the connection lock.  This way we are free to move the
auth_meta over to the new connection as long as we are holding the lock,
and at the same time the existing connection can fiddle with the old
auth_meta without being disturbed.  (That old auth_meta is about to get
discarded, but we still need to prevent the two threads from stomping on
each other.)

This also cleans up the reset_recv_state() a bit since we can simply
replace the old auth_meta with a totally fresh one without worrying about
what kind of state might be lurking in there.

Signed-off-by: Sage Weil <sage@redhat.com>