get_entries is now sending multiple queries.

Now get_entries can send multiple requests at the same time to fetch the entries (you can set how many entries are fetched at the same time). Also while testing it I found out that error headers are not attached in logs, so I squeezed that change here.  I removed one unit test for async client (one that checks if client returns partial result if range was too big), because currently all classes that use async log client care more about getting exception as soon as possible than getting partial result.
Nit: Another approach would be to count the number of available retries and going on until the number of available retries is 0. 
The very small benefit is that the number of retries is clear in the request code (in this case _create_fetch_deferred).
No need to change the code though.