Clarify Connection::space_can_send logic

[Ralith]

Testing and re-examination reveals that this doesn't actually change any behavior, because if ACKs were sendable then SendableFrames::acks would have been set by the first PacketSpace::can_send call. We only reach the case that hard-codes that to false if we already know it would have returned false. Cleaned up the logic a bunch to avoid future confusion; please review carefully for absence of semantic changes.

[djc]

Much easier to review, thanks!