add local flag to the socket object

[sleimanx2]

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behaviour

Currently, the only way to set the local flag when in socket context is as follow

socket.flags['local'] = true;

New behaviour

Now we can add the local flag the same way we do it on the Namespace object

socket.local...

Other information (e.g. related issues)

#3217

[darrachequesne]

Thanks!