More data in Dynamo, less on S3

[rclark]

Increase the amount of geobuf data that can be stored in individual Dynamo records from 10KB maybe-up-to 350KB. Also don't store geobufs on S3 unless it doesn't fit into the record.

Advantages:

• Making fewer S3 requests will improve read speed for most datasets

Disadvantages:

• This will significantly impact read capacity requirements on dataset listing functions and calculating dataset metdata
• Paginated .list() requests could return as few as 2 records per page.

cc @mick @willwhite

[rclark]

Next steps:

• make max-size in DynamoDB easily configurable
• only put to S3 for items that exceed DynamoDB max-size. We currently put everything to S3 regardless of whether or not it fits in DynamoDB

[rclark]

This is already halfway done: MAX_GEOMETRY_SIZE can be set via client config. The other half is #151.