java: extract abstraction of isolated application

We aim to support running multiple isolated Java applications in one
JVM. Some work has already been done to isolate system class
loaders. There is much more to it than that. Isolated applications
(aka Contexts) do not map 1-1 to class loaders. One context may have
many different class loaders.

This change extracts context-specific logic to separate classes as a
base for further additions.