Streaming data alarm evaluation

Just a initial idea to catch your attention :) the current alarm evaluation depend on the statistics API of ceilometer, the API will aggregate some metric data which searched in the whole metric data storage, that is inefficient. almost all the alarm evaluation only need the metric data during a latest period of time. So this proposal want to make Ceilometer also publish samples to aodh when publish them to ceilometer storage, and aodh will listen the sample (we already have a listener for event alarm), and cache the samples during a time window, the alarm evaluation base on these cached metric data will be efficient.