Use BMP End-of-RIB to trigger peer_up_withdraws_handler

[matthieugouel]

Right now we wait a random timer (5min +/- 1min) dropping update that has not been re-sent after a restart.
This is done in order to catch missing withdraws that could happen after the last state dump.

But we could use instead the End-of-RIB instead of a timer to more precisely know when to do this check.
https://datatracker.ietf.org/doc/html/rfc4724#section-2

An UPDATE message with no reachable Network Layer Reachability
   Information (NLRI) and empty withdrawn NLRI is specified as the End-
   of-RIB marker that can be used by a BGP speaker to indicate to its
   peer the completion of the initial routing update after the session
   is established.  For the IPv4 unicast address family, the End-of-RIB
   marker is an UPDATE message with the minimum length [[BGP-4](https://datatracker.ietf.org/doc/html/rfc4724#ref-BGP-4)].  For any
   other address family, it is an UPDATE message that contains only the
   MP_UNREACH_NLRI attribute [[BGP-MP](https://datatracker.ietf.org/doc/html/rfc4724#ref-BGP-MP)] with no withdrawn routes for that
   <AFI, SAFI>.