Use Index as Default Key

[rmorshea]

To get a basic understanding for what this means refer to React's documentation on this subject:

• Usages of keys - https://reactjs.org/docs/lists-and-keys.html
• Why keys are needed at all - https://reactjs.org/docs/reconciliation.html#keys

Task list:

• log when dynamic children do not have a key - add debug logs #377
• add warning indicating this will change in a future release - Add warning that element index will be used as the default key in a future release #428
• fix tests and make IDOM_FEATURE_INDEX_AS_DEFAULT_KEY=1 the default
• remove this feature flag.