Returning a customized CompletableFuture to replace standard CompletableFuture.

Instead of returning a standard CompletableFuture for an async call, consider returning a customized Future instance, for example, `DubboFuture extends CompletableFuture`, so that we can override the default behaviors of the Future, for example, cancel, get and so on.

What do you think?

Review the async call APIs that currently return CompletableFuture and compare the default cancel and get behavior with the proposed DubboFuture subclass. Clarify which behaviors should change and how callers should observe them; done requires an agreed design and corresponding tests for the selected Future operations.

Written by the indexing model from the issue text.