bgpd: Do not keep stale paths in Adj-RIB-Out if not addpath aware

bgpd: Do not keep stale paths in Adj-RIB-Out if not addpath aware

```
munet> r1 shi vtysh -c 'show ip bgp update advertised-routes'
update group 1, subgroup 1
BGP table version is 5, local router ID is 192.168.137.1
Status codes:  s suppressed, d damped, h history, u unsorted, * valid, > best, = multipath,
               i internal, r RIB-failure, S Stale, R Removed
Origin codes:  i - IGP, e - EGP, ? - incomplete
     Network          Next Hop            Metric LocPrf Weight Path
 *> 1.0.0.0/24       192.168.137.201                10      0 65200 65444 i
 *> 10.0.0.0/24      192.168.137.100                10      0 65100 65444 65444 i
 *> 10.65.10.0/24    192.168.137.100          0     10      0 65100 i
 *> 10.200.2.0/24    192.168.137.202          0     10      0 65200 i
```

Announce one more 10.0.0.0/24 via 65200 and we have TWO paths 10.0.0.0/24 in adj-rib-out:

```
munet> r1 shi vtysh -c 'show ip bgp update advertised-routes'
update group 1, subgroup 1
BGP table version is 6, local router ID is 192.168.137.1
Status codes:  s suppressed, d damped, h history, u unsorted, * valid, > best, = multipath,
               i internal, r RIB-failure, S Stale, R Removed
Origin codes:  i - IGP, e - EGP, ? - incomplete
     Network          Next Hop            Metric LocPrf Weight Path
 *> 1.0.0.0/24       192.168.137.201                10      0 65200 65444 i
 *> 10.0.0.0/24      192.168.137.100                10      0 65100 65444 65444 i
 *> 10.0.0.0/24      192.168.137.201                10      0 65200 65444 i
 *> 10.65.10.0/24    192.168.137.100          0     10      0 65100 i
 *> 10.200.2.0/24    192.168.137.202          0     10      0 65200 i
```

Stop announcing 10.0.0.0/24 via 65200 and we still have TWO paths for 10.0.0.0/24...

```
munet> r1 shi vtysh -c 'show ip bgp update advertised-routes'
update group 1, subgroup 1
BGP table version is 7, local router ID is 192.168.137.1
Status codes:  s suppressed, d damped, h history, u unsorted, * valid, > best, = multipath,
               i internal, r RIB-failure, S Stale, R Removed
Origin codes:  i - IGP, e - EGP, ? - incomplete
     Network          Next Hop            Metric LocPrf Weight Path
 *> 1.0.0.0/24       192.168.137.201                10      0 65200 65444 i
 *> 10.0.0.0/24      192.168.137.100                10      0 65100 65444 65444 i
 *> 10.0.0.0/24      192.168.137.201                10      0 65200 65444 i
 *> 10.65.10.0/24    192.168.137.100          0     10      0 65100 i
 *> 10.200.2.0/24    192.168.137.202          0     10      0 65200 i
```

Why do we need to keep old paths in adj-rib-out if we don't have e.g. AddPaths enabled?

Shouldn't it be like here? (only one 10.0.0.0/24 in adj-rib-out for this update-group instead of multiple (stale from previous announcements))

```
munet> r1 shi vtysh -c 'show ip bgp update advertised-routes'
update group 1, subgroup 1
BGP table version is 6, local router ID is 192.168.137.1
Status codes:  s suppressed, d damped, h history, u unsorted, * valid, > best, = multipath,
               i internal, r RIB-failure, S Stale, R Removed
Origin codes:  i - IGP, e - EGP, ? - incomplete
     Network          Next Hop            Metric LocPrf Weight Path
 *> 1.0.0.0/24       192.168.137.201                10      0 65200 65444 i
 *> 10.0.0.0/24      192.168.137.201                10      0 65200 65444 i
 *> 10.65.10.0/24    192.168.137.100          0     10      0 65100 i
 *> 10.200.2.0/24    192.168.137.202          0     10      0 65200 i
```

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