bgpd: Do not delete BGP dynamic peers if graceful restart kicks in

```
~# vtysh -c 'show bgp ipv4 unicast summary' | grep 192.168.10.17
*donatas-pc(192.168.10.17)           4      65002         8        12        0    0    0 00:01:35            2       14 N/A
```

Before shutting down 192.168.10.17:

```
~# vtysh -c 'show bgp ipv4 unicast 100.100.100.100/32'
BGP routing table entry for 100.100.100.100/32, version 7
Paths: (2 available, best #2, table default)
  Advertised to non peer-group peers:
  home-spine1.donatas.net(192.168.0.2)
  65002, (stale)
    192.168.10.17 from donatas-pc(192.168.10.17) (0.0.0.0)
      Origin incomplete, valid, external
      Last update: Sat Jan 15 21:45:47 2022
  65001
    192.168.0.2 from home-spine1.donatas.net(192.168.0.2) (2.2.2.2)
      Origin incomplete, metric 0, valid, external, best (Older Path)
      Last update: Sat Jan 15 21:25:19 2022
```

After 192.168.10.17 is down:

```
~# vtysh -c 'show bgp ipv4 unicast summary' | grep 192.168.10.17
donatas-pc(192.168.10.17)            4      65002         5         9        0    0    0 00:00:12       Active        0 N/A

~# vtysh -c 'show bgp ipv4 unicast 100.100.100.100/32'
BGP routing table entry for 100.100.100.100/32, version 7
Paths: (2 available, best #2, table default)
  Advertised to non peer-group peers:
  home-spine1.donatas.net(192.168.0.2)
  65002, (stale)
    192.168.10.17 from donatas-pc(192.168.10.17) (0.0.0.0)
      Origin incomplete, valid, external
      Community: llgr-stale
      Last update: Sat Jan 15 21:49:01 2022
      Time until Long-lived stale route deleted: 16
  65001
    192.168.0.2 from home-spine1.donatas.net(192.168.0.2) (2.2.2.2)
      Origin incomplete, metric 0, valid, external, best (First path received)
      Last update: Sat Jan 15 21:25:19 2022
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>

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