Add ability to skip placing unchanged output files

[dfederm]

This adds a new setting which keeps state for files that it places on disk from cache. On subsequent builds, it will read this state and avoid copying files which are unchanged.

A file is considered unchanged if:

• It exists
• The previously placed file and file to be placed have the same hash
• The previously placed file and current file on disk have the same timestamp
• The previously placed file and current file on disk have the same file size