Clearer openStream modes

[natevw]

Right now the whole 'rx'/'tx' mode stuff is hard to explain. It might be better to expose this as four distinct "modes":

• 'tx' — stream.Writable, allowing control over whether ack should be requested and whether failed ack should be an auto-closing error
• 'tx+rx' — stream.Duplex, but won't receive data unless data is being sent
• 'rx' — stream.Readable
• 'rx+tx' — stream.Duplex, but sends must buffer until data is received