DATAMONGO-929 - Use property path for keys of Indexed and CompoundIndex.

Index creation failed for `@Indexed` and `@CompoundIndex` as the resolved _dotPath_ was not used for creation. 

We now not only resolve the _dotPath_ but also use it within the key for index definition. In case of a nested compound index the key definition is enhanced by the provided path. 

When leaving the key definition empty for nested compound index we'll create an index for the whole nested document.

Trying to create a compound index on root level not providing key information leads to `InvalidDataApiUsageException`.