bgpd: Track RTRLib memory allocations separately

bgpd: Track RTRLib memory allocations separately

With fast `rpki reset`, seems that RTRLib related memory allocations leak
the memory. Need to figure this out later.

```
$ vtysh -c 'show memory | include RPKI'
BGP RPKI Cache server         :       12 variable       576       12       576
BGP RPKI Cache server group   :        0     48           0        1        56
BGP RPKI RTRLib               :   849657 variable  34453096   849662  41293504

$ for x in $(seq 1 60); do vtysh -c 'con' -c 'rpki' -c 'rpki reset'; sleep 1; done

$ vtysh -c 'show memory | include RPKI'
BGP RPKI Cache server         :       12 variable       576       12       576
BGP RPKI Cache server group   :        0     48           0        1        56
BGP RPKI RTRLib               :   849699 variable  69490616   849701  76323832
```

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