curl_json plugin: avoid cache aliasing by using all fields instead of the final two

The following two keys

  Key workers/*/requests
  Key workers/*/apps/*/requests

can both lead to type_instance's of say

  0-requests

which would alias in the cache.  If, instead of just the final two key
name components, we use all of them it would lead to these

  Key workers-0-requests
  Key workers-0-apps-0-requests

which would not overlap.
Merge pull request #408 from radford/curl-json-cache-aliasing

curl_json plugin: avoid cache aliasing by using all fields instead of just two