[FLINK-4434] [rpc] Add a testing RPC service.

[StephanEwen]

Adds an RPC Service implementation for testing. This RPC service acts as a replacement for the regular RPC service for cases where tests need to return prepared mock gateways instead of proper RPC gateways.

The TestingRpcService can be used for example in the following fashion, using Mockito for mocks and verification:

TestingRpcService rpc = new TestingRpcService();

ResourceManagerGateway testGateway = mock(ResourceManagerGateway.class);
rpc.registerGateway("myAddress", testGateway);

MyComponentToTest component = new MyComponentToTest();
component.triggerSomethingThatCallsTheGateway();

verify(testGateway, timeout(1000)).theTestMethod(any(UUID.class), anyString());

This also adds a utility to create futures with a predetermined timeout.

[tillrohrmann]

Changes look very good to me :-) Thanks for your work @StephanEwen. Will merge this PR.

[uce]

typoe teh => the

[uce]

Cool stuff! Really looking forward to when this is merged back to master some day. 👍

[StephanEwen]

Manually merged to the flip-6 branch in eee1497