add ResetCheckpoints function

[frrad]

Adds a ResetCheckpoints function which sets all existing dynamo checkpoints to LATEST.

This is useful to call as part of a deploy if you want to make sure that all new readers start at the most recent record instead of picking up where the the old readers left off.

[frrad]

Addressed your comments. Please take another look.

For my use case I am not slowed down too much by doing updateItem in just one goroutine, but happy to add more if you want. Let me know.