Updating .fits_runner -> .default_fits_runner
Explaining the "why" of the AntiVirus behavior

There are two collaborating classes that may not be aware of their
interaction: FileContentDatastream and CharacterizeJob.

The CharacterizeJob tells the FileContentDatastream to characterize the
file. Given the current architectural constraints, the
FileContentDatastream#characterize is also performing Anti-Virus checks,
so as to reduce the amount of network traffic.
Renaming fits_runner to characterization_runner

Fits is the technical implementation, characterization is the functional
requirement/behavior.