Double datastore measurements

We're measuring datastore puts, gets, etc. in two places:

1. In the config, we wrap each sub-datastore with a measure datastore.
2. In the fsrepo code, we wrap the final datastore in a global measure datastore.

On one hand, this gives us a nice global view into datastore stats. On the other hand, this seems like duplicate work:

* When we only have one datastore, we're just duplicating work.
* We should be able to get all the stats we're looking for by combining the measurements from both datastores (I think?).