Sanitize flowspec parsing

Sanitize flowspec parsing

Flowspec support performs a rather ugly indirection through Object[] to
deal with the fact that base and L3VPN routes have different structure.

The problem core issue here is lack of proper specialization, where
AbstractFlowspec{NlriParser,RIBSupport} really cater to plain Ipv4/Ipv6
supports -- and thus there is no simple-specific meeting point.

Introduce AbstractFlowspecIp{NlriParser,RIBSupport} to act as proper
specialization, which allows us to structure the code correctly.

This patch deals with the parsing path, leaving the serialization path
to a follow-up.

Change-Id: Ifdc3241c9f87c06eec112550103e67b26973c80f
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>