implemented streaming for tags retrieval

mostly a proof of concept for now
refactored data retrieval to improve separation of concerns

retrieval is now encapsulated in the `commands` module ("commands" here
refers to the design pattern, not twanager)

the goal is to keep dispatchers (i.e. both web handlers and twanager
commands) lean

the API isn't perfect yet though, WRT both naming and return values -
the implicit coupling between retrieving tagged tiddlers and related
tags is particularly awkward (cf. inline comments)