Xoto3

Fix regression in `backoff` and associated implementation.

`LazyMulticast` - a ContextManager-based interface for
process-local multicasting of a producer.

`funnel_latest_from_log_group` - CloudWatch Logs funnel that composes
easily with the above.

`funnel_sharded_stream` - Genericized sharded stream funnel that
guarantees receiving subsequent writes. Previously existing DynamoDB
streams processor `process_latest_from_stream` uses this now.

Cover more exception types in retries for DynamoDB transaction utilities.

Update `all_items_for_next_attempt` to be able to handle multiple
tables with different key schemas

`TypedTable` - An enhanced API supporting the use of
`dynamodb.write_versioned.versioned_transact_write_items`, which is a
great way to write business logic against DynamoDB.

Change all Resources and Clients to use a thread-local Session to
avoid race conditions.