bgpd: Update failed reason to distinguish some NHT scenarios

bgpd: Update failed reason to distinguish some NHT scenarios

Current failed reasons for bgp when you have a peer that
is not online yet is `Waiting for NHT`, even if NHT has
succeeded.  Add some code to differentiate this.

eva# show bgp ipv4 uni summ failed
BGP router identifier 192.168.201.135, local AS number 3923 vrf-id 0
BGP table version 0
RIB entries 0, using 0 bytes of memory
Peers 2, using 43 KiB of memory
Neighbor        EstdCnt DropCnt ResetTime Reason
192.168.44.1          0       0    never  Waiting for NHT
192.168.201.139       0       0    never  Waiting for Open to Succeed
Total number of neighbors 2
eva#

eva# show bgp nexthop
Current BGP nexthop cache:
 192.168.44.1 invalid, peer 192.168.44.1
  Must be Connected
  Last update: Mon Feb 10 19:05:19 2020

 192.168.201.139 valid [IGP metric 0], #paths 0, peer 192.168.201.139

So 192.168.201.139 is a peer for a connected route that has not been
created on .139, while 44.1 nexthop tracking has not succeeded yet.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>