Allow using setServerNames on WebSocket

[MinnDevelopment]

Currently, you have to use WebSocketFactory#setServerNames for SNI parameters. However, I want to use the factory for different connections to different domains. This requires doing synchronization on the factory only to set the server name:

synchronized (socketFactory) {
    String host = IOUtil.getHost(url);
    // null if the host is undefined, unlikely but we should handle it
    if (host != null)
        socketFactory.setServerName(host);
    else // practically should never happen
        socketFactory.setServerNames(null);
    socket = socketFactory.createSocket(url);
}

I have 3 solutions to this problem with decreasing complexity:

1. Allow setting the server names on the socket rather than only the factory, this would require a few changes to the handling of the raw socket creation
2. Allow passing the server names in createSocket to have an idempotent behavior
3. Add a way to copy the factory, this should be fairly trivial to implement. For instance new WebSocketFactory(otherFactory) or similar. I would be willing to PR this.

[TakahikoKawasaki]

@MinnDevelopment Thank you. I'm interested in what your PR will look like 😀 . Please note that I'm a bit picky about beauty of source codes, so PRs that "just work" won't be merged 😅 . My programming style can be found in repositories in github.com/authlete and github.com/TakahikoKawasaki. Finally, I'm sorry I'm busy as a co-founder and programmer of Authlete, Inc., so it may take time for me to respond.

[MinnDevelopment]

@TakahikoKawasaki I already have something but I haven't gotten around to testing it yet. I will try my best to adhere to the code style.

[TakahikoKawasaki]

@MinnDevelopment Thank you for your PR! nv-websocket-client 2.10 including your PR was released. I hope the release will appear in the Maven central repository soon.