Use build_key(key, namespace) consistently across modules and classes

[padraic-shafer]

• See build_key(key, namespace) is missing from some modules/classes #569 for more information
• Use namespace for decorators get/set
  • Include build_key(key, namespace) in decorators.cached:
    • get_from_cache()
  • set_in_cache()
• Use namespace for HitMissRatioPlugin key
  • Use namespace, if available, to build key that is passed
    to plugins.HitMissRatioPlugin.post_get()
• Introduce key_filter for confirming key is in namespace
  • BaseCache.key_filter(key, namespace) returns True
    if key is in the namespace
  • backends.memory.SimpleMemoryBackend uses this to seleectively
    clear its cache for the given namespace

What do these changes do?

Keys with namespaces now work throughout the aiocache package

• Use namespace for decorators get/set
• Use namespace for HitMissRatioPlugin key
• Introduce key_filter for confirming key is in namespace

Are there changes in behavior for the user?

Keys with namespaces now work throughout the aiocache package.
This includes using:

• the @cached() decorator
• the HitMissRatioPlugin plugin
• the SimpleMemoryCache.clear() member

Related issue number

#569: build_key(key, namespace) is missing from some modules/classes

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[padraic-shafer]

Hi, do you have an feedback for this PR?

[Dreamsorcerer]

Thanks, but will need a test that shows the incorrect behaviour.

Also, please update to latest master, thanks.