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.
Per our offline discussion: We can change the log client's behaviour to throw if the caller requests for entries beyond the tree size. In that case it's better to throw early than let a fetcher try to fetch the non-existent range,retry multiple times and fail.
It's OK, however, to re-try fetching entries that are in the tree (based on the tree size in the STH) and throw if the log does not provide them.