Add PCEPTopoloy{Instance,Singleton,Tracker}

Add PCEPTopoloy{Instance,Singleton,Tracker}

Rework topology instantiation to split the individual concerns.
PCEPTopologyTracker is responsible for finding out when a particular
topology becomes interesting to the underlying code (and when becomes
uninteresting).

For each such topology, a PCEPTopologySingleton is instantiated. It then
handles allocation of the underlying InstructionScheduler as well as
bootstrapping a cluster-wide singleton PCEPTopologyInstance to handle
that topology's configuration.

PCEPTopologyInstance tracks configuration datastore changes for a single
topology. For each update it constructs an immutable
PCEPTopologyConfiguration and forwards it to its PCEPTopologyProvider.

PCEPTopologyProvider handles updates of the configuration in the most
graceful manner possible, sequencing possible Netty Channel restart, but
otherwise just updaing the low-level componennts

JIRA: BGPCEP-893
Change-Id: I56c47b89ab87cf33a6be1016fa132b6c1aa98cbb
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>