Improve mvpn parsing and serialization

Improve mvpn parsing and serialization

Require MvpnSerializer implementations to provide a properly-specialized
class. Centralize implementations in AbstractMvpnNlri, so that we do not
get a virtual dispatch here.

Also specialize SimpleMvpnNlriRegistry to use ImmutableMap/EnumMap rather
than HandlerRegistry, which is using the overly-dynamic MultiRegistry --
which ends up speeding up lookups significantly.

Change-Id: I09058410b4b8c3042c6273fe470530594387a09e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>