Create Log Stream first instead of trying to get next sequence token

[amekkawi]

Related to #8, it may be better to take a different approach to handling the initial setup before pushing log events to a log stream.

Right now cwlogs-writable does this:

1. Describe the Log Stream to get the next sequence token
2. If it doesn't exist, create the Log Stream and go back to step 1

This means 2 DescribeLogStreams requests are sent for Log Streams that do not yet exist, which should be the case if you're using unique enough Log Stream names.

Instead it may be better to do this:

1. Attempt to create the Log Stream, ignoring failure
2. Describe the Log Stream to get the next sequence token (unless the create log stream response includes the token)