Cache paths for elastic search store

As pointed out in pyr/cyanite#35, the elastic search path store has to hit elasticsearch for each metric.

This was pretty bad, and more testing showed that it was a big bottleneck for writes.

This implements a simple cache so that after writing, only one check is needed. 

One thing to note, just realized that currently, writing paths to elasticsearch currently isn't tenant aware. This doesn't fix that, but expect a patch soon :)