Fix corner case in SchedulingGroup::UpdatePeerQueue logic

An assertion fails if a peer gets blocked when dequeueing updates from
multiple RibOuts via SchedulingGroup::UpdatePeer.

Problem happens in the following situation:

- Peer was previously blocked and now has updates to send for 2 RibOuts.
- Updates for both RibOuts are for the same queue i.e. QBULK or QUPDATE.
- Peer shares a marker for the for the first RibOut with another peer or
  peer's marker gets merged with marker for another peer when sending
  updates for first RibOut (via RibOutUpdates::PeerDequeue)
- There are still more updates to be sent for the first RibOut i.e. the
  processing in RibOutUpdates::PeerDequeue keeps going.
- Original peer gets send blocked, but we manage to dequeue all updates
  for the first RibOut to the other peer with which the original peer's
  marker got merged.
- RibOutUpdates::PeerDequeue returns true because of the previous point

At this point, we continue and try to dequeue updates for the 2nd RibOut
because RibOutUpdates::PeerDequeue returned success. We hit an assertion
in RibOutUpdates::PeerDequeue when called for the 2nd RibOut because the
original peer is not in the send ready set anymore.

Fix is to stop processing RibOuts for the peer if it's send blocked when
RibOutUpdates::PeerDequeue returns.  This ensures that we don't hit the
assertion since we don't try to process the 2nd RibOut.  Updates for the
2nd RibOut will be sent to the other peer when it's WorkPeer item gets
processed.

Change-Id: Ib1ef218ad9eecb1ca489b3045bdc3419e75caa21
Closes-Bug: 1386460