Add keys to the entity_import_status table

Problem/Motivation

First, thank you very much for your work on this great module!

I did a db profiling of an entity share run from on site to another. I found two queries which seemed quite slow, each one accounting for about 3 percent of the total mysql load.
I have only about 800 elements in the entity_import_status table - for larger datasets this could be even worse.
See example queries (queries.txt). These queries have no keys that they can use and must do a full table scan.

Steps to reproduce

Benchmark a big entity share with drush. The effect is bigger when you have a lot of simple entities without files - and a optimized settup using memcache or redis as a cache backend.

Add the keys by hand on master and client installation, see add_keys.txt.

Benchmark again.

Proposed resolution

Add the two keys.

Remaining tasks

Data model changes

This changes the db model. The db model would also profit from using smaller field definitions, e.g. the same the node table uses (uuid is varchar 128, ascii_general_ci).