Update the dynamodb session handler docs to suggest using TTL rather than manual garbage collection

[jonathonsim]

Now that DynamoDB supports Time to Live, this is a much better solution than manual garbage collection for expiring sessions. The session expires attribute is already in a suitable format for this. The suggested update points this out rather than giving users the (possibly bad) advice to setup garbage collection.

Also added some hints around how to setup the table, and suggest the on-demand capacity mode as an alternative to trying to guess the right provisioned capacity.

These are all relatively recent features in DynamoDB, but these are settings we are running in production for our clients at Idealstack and they seem to work well.

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[jschwarzwalder]

Thank you for your submission. We might edit the content after merging, but appreciate the time you took in sharing your knowledge.

[jonathonsim]

No worries on edits. Found myself writing in a blog post how the SDK docs were wrong on this point and just thought that a PR was probably a better way to deal with it :-)