Bug fixes associated with readBatch() and writeBatch()

 - Fixed a bug in pixii.Table that would cause elements of the input
iterator to be skipped in writeBatch() if an exception triggered the
retry policy.
 - Fixed a bug in pixii.fake.FakeDynamoDb that was incorrectly throwing
exceptions when an item in a batch get request was not found.
 - Fixed a bug in pixii.fake.FakeTableWithHashKey that was causing
elements to be stored under the wrong key when inserted using putItem().