Revert "Simplify the dispatcher method prototype"

This reverts commit fe1158de8ca5f70fa219da706d4960b864654be7.
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.

Conflicts:
	etc/ceilometer/ceilometer.conf.sample
	setup.cfg
Spool metering messages to reduce mq connections