maintain `list series filtered by regex` in cache.
maintain `list series filtered by regex` in cache.

instead of merely caching the global list, and using that
as input to generate leaves and branches, we now maintain
the subset of the global list that matches the regex,
and use that to make generation of leaves and branches much
faster.

InfluxGraph/influxgraph,InfluxGraph/influxgraph,vimeo/graphite-influxdb,raintank/graphite-influxdb