Issue #214 Less agressive sandbox termination

- Sample at least a full channel's worth of messages before being able to
terminate a sandbox.  This slows down detection but greatly reduces false
positives.  This applies to the matcher and filter channels.

- Prevent a backpressured condition from causing severe oversampling of message
processing times.

- Remove the sampling after a timer event, it can be used to game the system by
returning quickly from the next process_message call.

- Rename MatcherAvgDuration to MatchAvgDuration to be consistent with the other
report attribute names.

- Add additional data to the blocked termination message so the exact cause is
clear.
Make the duration sample rate a constant
Merge pull request #217 from mozilla-services/issue_214

Issue #214 Less aggressive sandbox termination