Preview release with API changes

CloudWatch Logs API support improvements.

• Add Limit field to QuerySpec and improve documentation.
• Fix and document behavior when there's no @ptr field available, e.g. under stats command.
• Fix and document Preview behavior and introduce special @deleted field to communicate when a preview result was deleted from the final result set.
• Support @deleted field in Unmarshal.

Other improvements.

• Fix deadlock and concurrency bugs in both main code and tests.
• Add runnable test examples for Unmarshal.
• Add framework for describing query test scenarios, and add happy path query tests.
• Fix miscellaneous bugs found while testing.