Xoto3

- Update `find_index` and `require_index` to allow for an index with no range key.

- Fixed `versioned_diffed_update_item` to support concurrent use with
transactions.

- Fixed bug in `versioned_transact_write_items` where un-effected
items would have an incorrect ConditionExpression generated, causing
the transaction to fail.

- `versioned_transact_write_items` now allows the transaction builder
itself to lazily load items from any table via `get`/`require`,
instead of requiring all transacted items to be explicitly declared
before the builder is invoked.
- Experimental support for optimistic creations and deletions as
well - if an item being written has not been prefetched, it will be
assumed to not exist for the purposes of the condition check in the
Put or Delete within the transaction. If that optimism proves
invalid, the transaction will be retried after a refetch of the
actual item. The limitation is that we need to be able to somehow
derive a key for your item, so either you need to have prefetched a
different item from the same table, or your environment must allow
access to the DescribeTable action for the DynamoDB table, so that
we can directly check its key schema.

- New `versioned_transact_write_items` wrapper that presents a general
purpose interface for versioned transactional Puts and Deletes
across tables and items, reverting to simple versioned writes for
cost optimization when only a single item is transacted.

- Improved utilities and types for CloudWatch Metrics.