Split streaming API from `WebClient`

If a user receives a fully aggregated HTTP response, he or she should call aggregate():

WebClient client = ...;
CompleteFuture<AggregateHttpResponse> response = client.get("/some/path").aggregate();

It could be boilerplate to call aggregate() every time because most REST API returns a unary HTTP response. I think we can improve this API design by splitting WebClient and StreamWebClient. The original idea was suggested by @trustin.

WebClient client = ...;
// Returns an aggregated response directly
CompleteFuture<AggregatedHttpResponse> response = client.get("/some/path");

// Switch to a streaming client. 
StreamingWebClient streamingClient = client.streaming();
// Returns an HttpResponse that can be subscribed by a Reactive Streams `Subscriber`
HttpResponse response = streamingClient.get("/some/path");

Furthermore, we can also provide a blocking WebClient for blocking services and tests.

WebClient client = ...;
// Switch to a streaming client. 
BlockingWebClient blockingClient = client.blocking();
// Wait for an aggregated response until fully received.
AggregatedHttpResponse response = blockingClient.get("/some/path");

[anuraaga]

Sounds very familiar 😛 #2364

Don’t think it needs to wait for 2.0.0, can leave WebClient as is and convert it to the wrappers as per @trustin’s suggestion

[anuraaga]

Another one or two years sounds better 😃 It is also a good reason to go with your proposed approach from #2364 I think as it does sound reasonable far off then (good thing).