Move Client.Watch inside Client.Attach and hide it from external interface

[hackerwins]

Description:

Move Client.Watch inside Client.Attach and hide it from the external interface.

Go SDK is just used in integration tests of servers without other SDK installations. So it was OK to expose Client.Watch to the external interface. But by adding more and more features to the SDK, it is quite difficult to keep simple tests.

Let's move Client.Watch inside Client.Attach and hide it from the external interface to maintain consistency with other SDKs and simplify testing.

Why:

Keep the product simple

[karockai]

It looks interesting. May I work on this issue?

[krapie]

@karockai sure! give it a try!

[krapie]

Do we need to change the entire interface to perform runWatchLoop() and provide subscribe() interface just like other SDKs?

[chacha912]

@krapie Yes, that seems necessary.
Here's the discussion we had about what tasks need to be carried out regarding this issue, for your reference.
https://codepair.yorkie.dev/community/65e02d3166a65e76f6703d15/share?token=2ij9re

[hackerwins]

Following #803, we need to apply the below features:

• Reconnection logic for watch connection failure.
• Apply SyncMode introduced in the JS SDK.

yorkie-team/yorkie-js-sdk#772