Review all these *Client receiver methods. They should probably either
all be receivers, or none.

Maybe put auth-related stuff into its own structure inside Client,
instead of at Client's top level.

Rename stanzaHandler to something like idCallback. Same for the
HandleStanza function.

Replace inputControl with something like an enum.

Add a way to broadcast status information as negotiation happens or
disconnects occur. Possibly a new type of object that can be sent on
Recv along with stanzas. Or use sync.Cond to protect a state
variable.

Asynchronously updating Client.Features is not thread safe.

NewClient shouldn't be asynchronous.

Add a Reconnect() function.

Put roster and bind into separate packages, if possible.

Eliminate as many uses of Generic as possible.