Correctly cache semanticType (fix undefined categories out of cache)

[tokebe]

(fixes biothings/biothings_explorer#234)

This change causes cache_handler to copy the semanticType of input/output nodes while copying records from a query response. When pulled from the cache, these values are read to give each kg node a category. Previously, none would be found and categories would become biolink:undefined.

Example query (test with caching enabled):

{
  "message": {
    "query_graph": {
      "nodes": {
        "n0": {
          "categories": [
            "biolink:Disease"
          ]
        },
        "n1": {
          "ids": [
            "HGNC:6284"
          ],
          "categories": [
            "biolink:Gene"
          ]
        }
      },
      "edges": {
        "e0": {
          "subject": "n0",
          "object": "n1"
        }
      }
    }
  }
}