Convert bgp-flowspec to OSGi DS

Convert bgp-flowspec to OSGi DS

As it turns out, FlowspecActivator is just a bunch of static wiring,
which we do not need to inject. Refactor SimpleFlowspecTypeRegistry
into a dedicated immutable FlowspecTypeRegistry, which has provably
static lifecycle and used only for immutable access.

This cuts a ton of complexity and fixes ServiceLoader injection,
plus makes DI conversion a breeze -- eliminating static state in
process of doing so.

JIRA: BGPCEP-941
Change-Id: I0ad4fef346131b44f7f5bca2a637826e821146d7
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>