better index support for AttributeMap #357

An indexed AttributeMap must use an explicit index key. It is not an index over all Map instances. Each instance write updates the same search field, so one instance can replace the value written by another instance. The Map instance key is not searchable.

This does not turn the Map into a KEYWORD_ARRAY. KEYWORD_ARRAY indexes the string array from one write; Dex does not combine the values from all Map instances. After the example below writes the payment instance with value authorized, you can query the current search value with: