bgpd: Stop LLGR timer when the connection is established

bgpd: Stop LLGR timer when the connection is established

When the connection goes up, the timer is not stopped and if we have a
subsequent GR event we have an old timer which is not as we expect.

Before:
```
spine1-debian-11# sh ip bgp 192.168.100.1/32
BGP routing table entry for 192.168.100.1/32, version 95
Paths: (1 available, best #1, table default, mark routes to be retained for a longer time. Requires support for Long-lived BGP Graceful Restart)
  Not advertised to any peer
  65001 47583, (stale)
    192.168.0.1 from 192.168.0.1 (100.100.200.100)
      Origin incomplete, valid, external, best (First path received)
      Community: llgr-stale
      Last update: Mon Mar 28 08:27:53 2022
      Time until Long-lived stale route deleted: 23 <<<<<<<<<<<<

spine1-debian-11# sh ip bgp 192.168.100.1/32
BGP routing table entry for 192.168.100.1/32, version 103
Paths: (1 available, best #1, table default)
  Advertised to non peer-group peers:
  192.168.0.1
  65001 47583
    192.168.0.1 from 192.168.0.1 (100.100.200.100)
      Origin incomplete, valid, external, best (First path received)
      Last update: Mon Mar 28 08:43:29 2022

spine1-debian-11# sh ip bgp 192.168.100.1/32
BGP routing table entry for 192.168.100.1/32, version 103
Paths: (1 available, best #1, table default, mark routes to be retained for a longer time. Requires support for Long-lived BGP Graceful Restart)
  Not advertised to any peer
  65001 47583, (stale)
    192.168.0.1 from 192.168.0.1 (100.100.200.100)
      Origin incomplete, valid, external, best (First path received)
      Community: llgr-stale
      Last update: Mon Mar 28 08:43:30 2022
      Time until Long-lived stale route deleted: 17 <<<<<<<<<<<<<<<
```

After:
```
spine1-debian-11# sh ip bgp 192.168.100.1/32
BGP routing table entry for 192.168.100.1/32, version 79
Paths: (1 available, best #1, table default, mark routes to be retained for a longer time. Requires support for Long-lived BGP Graceful Restart)
  Not advertised to any peer
  65001 47583, (stale)
    192.168.0.1 from 192.168.0.1 (0.0.0.0)
      Origin incomplete, valid, external, best (First path received)
      Community: llgr-stale
      Last update: Mon Mar 28 09:05:18 2022
      Time until Long-lived stale route deleted: 24 <<<<<<<<<<<<<<<

spine1-debian-11# sh ip bgp 192.168.100.1/32
BGP routing table entry for 192.168.100.1/32, version 87
Paths: (1 available, best #1, table default)
  Advertised to non peer-group peers:
  192.168.0.1
  65001 47583
    192.168.0.1 from 192.168.0.1 (100.100.200.100)
      Origin incomplete, valid, external, best (First path received)
      Last update: Mon Mar 28 09:05:25 2022

spine1-debian-11# sh ip bgp 192.168.100.1/32
BGP routing table entry for 192.168.100.1/32, version 87
Paths: (1 available, best #1, table default, mark routes to be retained for a longer time. Requires support for Long-lived BGP Graceful Restart)
  Not advertised to any peer
  65001 47583, (stale)
    192.168.0.1 from 192.168.0.1 (100.100.200.100)
      Origin incomplete, valid, external, best (First path received)
      Community: llgr-stale
      Last update: Mon Mar 28 09:05:29 2022
      Time until Long-lived stale route deleted: 29 <<<<<<<<<<<<<<
```

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