Avoid creation of new software release state change packing list only if
similar one is the topmost one.

Cache for list is not implemented and in order to avoid hiding other problems
provide no cache variant of test.

Test runners cannot use pure in-ZODB tests.

Also reuse getAPIRoot

API is not an object, so generate virtual URLs.

Convert tool into container of various versions of API.
Move API to Document and use portal_components.

The default exchange format is application/json and currently the API is
supposed to use only this format, so simplify the communication.

The API shall be simple, yet powerful. Introspection seemed like cool idea, but
is not required.

There are limited amount of endpoints. There are few parameters. Those things
shall be documented.

Backward compatibility is easy. Adding new endpoint or parameter shall be always
optional.

In case of rewriting versioning will be used.