Reset online clients when stream is disconnected

[chacha912]

What happened:
Currently, when the stream is disconnected, the information regarding online-clients is not reset as expected. Instead, this information persists even after disconnection.

What you expected to happen:
When the stream is disconnected, online-clients' information should be cleared and no longer accessible.

How to reproduce it (as minimally and precisely as possible):

Execute the example quill-two-clients.html

reset.mov

Anything else we need to know?:

Environment:

• Operating system:
• Browser and version:
• Yorkie version (use yorkie version):
• Yorkie JS SDK version: