Butler: add support for write-once-compare-same outputs

Want to be able to write to a file and be sure that the file is 1. locked and 2. if that content exists, make sure that it is the same (and if it's not the same then respond - no op, or throw)

Examples of target files are policy (soon to be "formerly paf", see DM-4171), and config, and schema files.

write function should look like the butler.put method, but also take a comparison method. Comparison could be optional and default to __eq__.

(this existed in trac: https://dev.lsstcorp.org/trac/ticket/2789)