implement basic infrastructure for scoped cache

This commit adds support for scoped cache, e.g. keys can be tagged
with a scope, so that the same key can exist in multiple scope and
returns the value based on the scope set.

This is practically requires for scoping by subnet in ECS, but
it doesn't implement ECS completely. This is just a framework
to make something like ECS possible in a module.

The scope search is currently non-exhaustive, it either returns
a value bound to given scope or look into global scope, nothing
in between.