Multi-threaded reader for Cloudwatch Logs

[bbayles]

PR #40 added a multi-threaded reading capability to the S3 reader. This PR adds the same to the Cloudwatch Logs reader.

When thread_count is nonzero, the reader will:

• Retrieve the log streams that have events in the requested time window
• Set up a thread pool with the number of threads requested
• Read from each log stream in a separate thread