Implement partitioning

[dturanski]

Partition selector may be configured in as an existing function name (in the module path), a lambda expression(powerful but not secure), or JSON path expression. If interoperating with apps written in other languages (e.g. Java) plain text, JSON, or XML str payloads are the best choice. It would be nice to provide good JSON path support OOTB. Default partition selector can use Python built-in hash()