Add Typed edges

Adds strictly typed edges to the pod, all edges now need to defined in the schema as ItemEdgeType. checks edges when calling insert_edge

• Add ItemEdgeType:

{
  "type": "ItemEdgeType", /* exactly this, and nothing else */
  "edgeName": "yourEdge", /* e.g. "author" */
  "sourceType": "YourSourceType", /* A valid item type, e.g. "Message" */
  "targetType": "YourTargetType", /* A valid item type, e.g. "Person"*/
}

• Add type_edges: HashMap<String, HashMap<String, String>> to schema.Schema, records all edge types on each item
• database_utils::insert_edge checks for both source and target types, and inserts if edge is defined in schema::Schema