performance benchmarks

[jameslamb]

The changes introduced in #51 (thanks again @wdearden !) did not impact any user code or change the algorithmic correctness of uptasticsearch. They did, however, substantially improve the speed and efficiency of the package.

Verifying that this change actually did what it said it would was tricky. We had to manually try the PR branch on our own machines and against other instances of ES and use system.time() manually to check the speed benchmarks.

Would love if someone would take a shot at adding performance benchmarks to our tests for CI! I would like to test the following:

• peak memory usage
• total execution time

This will be pretty difficult, I think, because of this library's reliance on connecting to a separate service over a network connection. I envision these tests being limited to the processing of data once it is returned from the server.