Threadsafe option to avoid mutex around API calls

When running inside a multi-threaded server such as EventMachine, and using a threadsafe HTTP library such as faraday, the mutex synchronization around API calls to the server can be avoided.

This commit adds the http_threadsafe option, and a set of tests that attempt to exercise longer running API calls such as index creations inside threads. The tests are in large part a copy of the existing stretcher_server_spec with unrelated tests removed and an attempt to create a second index at the same time as the first is being created.

Let me know if this meets your standards for incorporating into the main code. Happy to make changes if required.