bgpd: Show the reason when the session is killed due to RTT

Simulated latency with:

```
tc qdisc add dev eth3 root netem delay 100ms
```

```
donatas-laptop# sh ip bgp summary failed

IPv4 Unicast Summary (VRF default):
BGP router identifier 192.0.2.252, local AS number 65000 vrf-id 0
BGP table version 28
RIB entries 0, using 0 bytes of memory
Peers 1, using 724 KiB of memory

Neighbor        EstdCnt DropCnt ResetTime Reason
192.168.10.65         2       2  00:00:17 Admin. shutdown (RTT)

Displayed neighbors 1
Total number of neighbors 1
donatas-laptop#
```

Another end received:

```
%NOTIFICATION: received from neighbor 192.168.10.17 6/2 (Cease/Administrative Shutdown) "shutdown due to high round-trip-time (104ms > 5ms, hit 21 times)"
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

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