RedisServer is ready support

[penev-ff]

Currently, the RedisServer class lacks a convenient mechanism to notify external code when the server has fully started and is ready to accept requests. I would be happy if a method or mechanism is introduced in the RedisServer class that signals when the server has completed its startup and initialization processes.

Example.

// Example if server is already running:
server.stop();
// - Here assert request that are using the mock server fails
server.start();
Awaitility.await().until(server::isReady);
// Proceed with assertions or operations requiring the server to be operational

[inponomarev]

1. Do I get this right that you are getting the connection error after server.start() as the connection attempt is made before the port is open?
2. What do you think if server.start() will simply block until the server is ready instead?

[inponomarev]

Ok I have investigated that if tests are run in the same thread, server is ready to accept connections after start().

However, there are two corner case usages for isReady:

1. If server is started in one thread, but is being connect from a separate thread.
2. If user "forgets" about the state of the server, e. g. doesn't know if start or stop has been called.

I have implemented isRunning method in #483

[penev-ff]

Thanks! 👍