Implemented wallaroo_tree:del_path/3
WIP on cascaded delete
Internal delete support smoke-tested
Minor fix to delete_entity/3 API func
REST: generic deletion support
Internal API handles deleting subsystems
added delete_check for tags
REST API support for entity deletion
REST support for deleting tags and branches
Deleting a node deletes its identity group.
py:  delete infrastructure support
py: Store#remove... methods
rb: remove* support
Support for deleting Wallaby entities.

Merging 'feature/delete-entities' brings the following functionality to Wallaroo:

1.  Data structure support for entity deletion (viz. wallaroo_tree, our versioned trie),
2.  Internal API support for deletion,
3.  REST API support for entity deletion, and
4.  Python and Ruby client library support for deletion.