Introduce restconf.server.{api,spi,mdsal}

Introduce restconf.server.{api,spi,mdsal}

This patch refactors restconf-nb wiring by introducing:
- RestconfServer, capable of invokeRpc()
- RestconfStreamRegistry, being the public interface to ListenersBroker
- RpcImplementation, being a native RestconfServer RPC invocation interface

This allows us to split out RPCs and logic for individual
RestconfStream.Sources -- which now become normal components injected
into MdsalRestconfServer.

This necessitates dealing with the fact that ListenerBroker requires
configuration. We take this on by splitting Config Admin interactions
into OSGiNorthbound, which then instantiates components based on
configuration.

After the dust settles, we end up ditching a controller dependency and
have a pretty crisp model of what gets done where and how.

JIRA: NETCONF-773
Change-Id: I3ea139dd811b487d676353adf075dcb102abc0c5
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>