Add an RPC collector dispatcher

To support cells, ceilometer collectors need to form a
heirarchy where messages are forwarded upwards to a collector
at the API level.

This implements the dispatcher interface using oslo.rpc.proxy.RpcProxy
for message forwarding to a given transport_url. Multiple dispatchers
are still supported, so a compute-level collector may still store
samples in its own database if desired.
Spool metering messages to reduce mq connections