pocket database #256

Currently, a database server is needed for level 3 delegation. A pocket database module can be useful for potential applications of Seamless that are unobtrusive, i.e. that prefer result files with explicit file names for storage, but that still wish to benefit from Seamless caching/reproducibility features.

Resist the temptation to make a pocket buffer-write server as well. Vaults will do, or an existing result file can be checksummed to see if there is a cache hit.

Preparation work is to split off the database models from database.py into a separate file.

The, actual implementation can be done initially within seamless-recipes.