bgpd: handle bgp pbr hash list destroy upon BGP destroy

bgpd: handle bgp pbr hash list destroy upon BGP destroy

Upon BGP destroy, the hash list related to PBR are removed.
The pbr_match entries, as well as the contained pbr_match_entries
entries.
Then the pbr_action entries. The order is important, since the former
are referencing pbr_action. So the references must be removed, prior to
remove pbr action.
Also, the zebra associated contexts are removed.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>