topotests: bgp_bmp, add a test to check for bgp vrf peer loc-rib message

topotests: bgp_bmp, add a test to check for bgp vrf peer loc-rib message

Add a test where, when the vrf interface is flapping, a peer down and a
peer up message are sent. This test, when used with ASAN, detects the
memory leak of the open_tx and open_rx messages of the loc-rib.

Refresh the method of updating the SEQ value when reading the peer
messages: only update to the last matching SEQ value.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>