BUG-7976: Race between peer removal and routes update

BUG-7976: Race between peer removal and routes update

There is a race condition when we are advertizing routes,
and a session peer goes down which will end in
removing the peer from DS.
Since advertizement and removal are done by 2 different tx,
we need to do it blocking mode, and not route should be
advertized until peer is removed and vice versa, peer
should not be removed until routes are advertized.
Otherwise toure update will try to update a peer/path which
is not longer present.
Fix by rework PeerExportGroup & ExportPolicyPeerTracker,
now instead of generate PeerExportGroup each time a peer
is reg/unreg,  we make PeerExportGroup work as a
blocking registry to solve previous issue.

Change-Id: I46a27871bfa2aa2a632e3bfb76061105c62bb6d2
Signed-off-by: Claudio D. Gasparini <claudio.gasparini@pantheon.tech>