refactor the switch-statement behavior in butler that serializes to different formats for fits, pickle, etc into a generic pluggable dispatch framework.

affects code in daf_persistence/src (c++ that we want to get rid of)
daf_persistence python; hard coded stuff in butler
potentially affects bypass_* functions in the mapper classes.

We must consider that transport protocol is a factor in this, targets include:

• stream
• disk
• database
• in-memory storage

We need to do some design work and will need to create stories for implementing the different targets.