Before this change, if one connection was authenticating this would
block any others from authenticating.
This was due to ssh.NewServerConn not being called in a go routine
after the Accept call.
This is fixed by running the ssh authentication in a go routine.
Thanks to @FiloSottile for advice on how to fix this.
See: https://github.com/golang/go/issues/43521
This is done by making fs.Config private and attaching it to the
context instead.
The Config should be obtained with fs.GetConfig and fs.AddConfig
should be used to get a new mutable config that can be changed.
This adds a context.Context parameter to NewFs and related calls.
This is necessary as part of reading config from the context -
backends need to be able to read the global config.
