Fetch the metrics utility from thread-local data

If the cache events do not have access to the current request object,
get the current metrics utility directly from pyramid_metrics, which
would get it from a thread-local data structure.
Workaround bug in webob.Response.from_file

Convert Response headers to str instances instead of unicode since it is
what WSGI expects.
Merge pull request #10 from deuxpi/webob-str-headers

Workaround bug in webob.Response.from_file
Style
Implement master switch to globally disable caching
Improve tests for redis key_versioner
Move key_versioner / redis master_version check into separate method
Handle condition where the master_version is set successfully but is still unavailable
Merge pull request #9 from pior/feature-master-switch

Feature master switch
Merge pull request #8 from deuxpi/hit-miss-events

Fetch the metrics utility from thread-local data