Issue #241 More accurate sandbox profiling

- Add a sandbox profile configuration option to gather a larger set of
ProccessMessage timings

- Add mutexes around the duration calculations since the numerator/denominator
could be updated during the calculation.

- Change the durations to int64 since it requires less conversions in the code

- update the sandbox configuration docs

- add the default MessageMatcher for the DashboardOutput
Merge pull request #243 from mozilla-services/issue_241

Issue #241 More accurate sandbox profiling