[bee #48, #49, #40]: introduce netstore

[acud]

Due to popular demand - the return of the immortal NetStore

This is a naive and trivial NetStore implementation: no caching and no fancy stuff, including any concurrency.

A note about about the retrieval protocol:
I introduced the Interface type but I cannot change the New constructor to return the Interface type, that is because in the retrieval tests, the server is expected to implement the Protocol method:

   recorder := streamtest.New(
   	streamtest.WithProtocols(server.Protocol()),
   )

Personally it would be nice to return the interface directly from the constructor and not even export Service at all, but then the test won't fly in this form.
It does work when I cast server to a Service:

	sprot := server.(*retrieval.Service)
	recorder := streamtest.New(
		streamtest.WithProtocols(sprot.Protocol()),
	)

But I feel this solution is also sub-optimal since Service still needs to be exported.
If someone has some nicer idea, I'd be happy to hear.

needed for #48
closes #49
closes #40
related to #98

[pradovic]

@acud You can always export something (constructor returning Service, for ex) only for tests in export_test.go. But how would you use Service if it is not exported? It is needed to add protocol to p2p, right? Sorry if I confused something.

[janos]

An extremely minor suggestion, m receiver is not needed.

[acud]

but it has to implement the interface (the validator is an interface, not a function type alias)

[janos]

This is very very minor, but just as swarm.Chunk is _, func (_ mockValidator) ... can have _.

[pradovic]

Lgtm 👏