registry backends that can leverage shared memory like reddis or memcached

[mikofski]

current: currently the default registry are in-memory dictionaries.

proposed: abstract the default into a backend to make registry backends extensible, allow different registry backends like memcached or reddis or even in memory sqlite3 to be used.

using shared memory will be more advantages for distributed calculations. for example if the dag of a calculation shows there are many orthogonal steps then the simulation can be distributed but the data, output and formula registries would have to be managed centrally.