Change FileOutput and TcpOutput to default to use_framing = true if a
ProtobufEncoder is being used, use_framing = false if any other
decoder is being used. The setting can still be overridden by explicitly
setting it in the config.
Remove outdated `prefix_ts` documentation, fix bug in use_framing
config processing.
Add a test to verify that the OutputRunner actually adds framing when
it has been told to do so.
Merge pull request #940 from mozilla-services/tcp-file-framing-default

Change FileOutput and TcpOutput to default to use_framing = true if a