#122 Fix unit tests failed

[TuanHungVU1202]

Fixes #122 with the suggested approach using CompletableFuture

[tuhuynh27]

@TuanHungVU1202 It's fine to let these codes sleep:

https://github.com/keva-dev/keva/blob/develop/core/src/test/java/dev/keva/core/server/AbstractServerTest.java#L196-L199

Because it's not about to wait for the server to start, it's about waiting for the command to execute

[tuhuynh27]

@yampml @axblueblader @haphananhtuan Please help to review

[tuhuynh27]

LGTM

[taivtse]

Hi @TuanHungVU1202, based on your idea, I have a suggestion like the one below.
In KevaServer, we will have an await method like this:

    @Override
    public void await() {
        ready.join();    // your ready future
    }

In where we want to wait until the server is available (e.g.: AbstractServerTest), just add this:

server.await();

[tuhuynh27]

@taivtse Nice idea, updated