!act #3581 Add Tcp.CompoundWrite, some cleanup

Moves `def ack: Event` and `def wantsAck: Boolean` from the `Tcp.WriteCommand` type down to the newly introduced `Tcp.CompactWriteCommand`, which breaks existing code depending on these.
Additionally `Tcp.WriteCommand` now has an additional method `def +:(other: CompactWriteCommand): CompoundWrite`.