Throttling or slow performance when searching group with many streams

When performing an `awslogs get` operation on a log group with many streams, even if I I specify the full log stream name I want to search, the performance is either very slow, or if the group has enough streams, I get a `ThrottlingException` error.

I've run into this when searching logs from AWS Batch. Batch uses the same log group for all jobs, `/aws/batch/jobs`, and puts the output from each job into its own stream. This means the `/aws/batch/jobs` log group ends up with a large number of streams if you use batch a lot. However, this shouldn't be a problem if I know the log stream I want to search.

For example, if I ran

```
awslogs get -GS -s 1d /aws/batch/job my-job/default/309e41b6173e4bb98171fb3529a58092
```

where the last argument is the complete log stream name, I would have expected fast performance since it doesn't need to search all log streams. However what the code actually does is treat the stream name as a regex, and list every log stream in the group and compare its name to the given pattern. This causes a `ThrottlingException` for me. In other instance where the group doesn't have quite so many streams, the problem manifests as just very slow performance.

Here is the error I get when I run the above command:

```
Traceback (most recent call last):
File "/Users/ajenkins/.local/bin/awslogs", line 8, in
sys.exit(main())
File "/Users/ajenkins/.local/pipx/venvs/awslogs/lib/python3.6/site-packages/awslogs/bin.py", line 179, in main
getattr(logs, options.func)()
File "/Users/ajenkins/.local/pipx/venvs/awslogs/lib/python3.6/site-packages/awslogs/core.py", line 109, in list_logs
streams = list(self._get_streams_from_pattern(self.log_group_name, self.log_stream_name))
File "/Users/ajenkins/.local/pipx/venvs/awslogs/lib/python3.6/site-packages/awslogs/core.py", line 102, in _get_streams_from_pattern
for stream in self.get_streams(group):
File "/Users/ajenkins/.local/pipx/venvs/awslogs/lib/python3.6/site-packages/awslogs/core.py", line 261, in get_streams
for page in paginator.paginate(**kwargs):
File "/Users/ajenkins/.local/pipx/venvs/awslogs/lib/python3.6/site-packages/botocore/paginate.py", line 255, in __iter__
response = self._make_request(current_kwargs)
File "/Users/ajenkins/.local/pipx/venvs/awslogs/lib/python3.6/site-packages/botocore/paginate.py", line 332, in _make_request
return self._method(**current_kwargs)
File "/Users/ajenkins/.local/pipx/venvs/awslogs/lib/python3.6/site-packages/botocore/client.py", line 357, in _api_call
return self._make_api_call(operation_name, kwargs)
File "/Users/ajenkins/.local/pipx/venvs/awslogs/lib/python3.6/site-packages/botocore/client.py", line 661, in _make_api_call
raise error_class(parsed_response, operation_name)
botocore.exceptions.ClientError: An error occurred (ThrottlingException) when calling the DescribeLogStreams operation (reached max retries: 4): Rate exceeded
```

I've just created a pull request to fix this. I added a `--stream-prefix` option to `awslogs get`, which tells it to treat the log stream argument as a string prefix instead of as a regex. Then it can pass it as the `logStreamNamePrefix` argument to `describe_log_streams`, which results in much faster performance, since the filtering is done in AWS. This completely fixes the problem for me.

Start in awslogs/core.py, especially _get_streams_from_pattern and get_streams, which the traceback identifies as listing and filtering streams. Check the existing pull request's --stream-prefix approach against awslogs get with a complete stream name; done means AWS performs prefix filtering without scanning every stream or triggering throttling.

Written by the indexing model from the issue text.