+act #3581 Add Tcp.CompoundWrite, some cleanup

Introduces `Tcp.SimpleWriteCommand` trait and `Tcp.CompactWrite` case class.
Additionally `Tcp.WriteCommand` now has a three additional methods (`+:`, `++:`, `prepend`) and a companion object.
=doc #3581 Add "Writing to a connection" section to IO docs
=pro #3581 Add mima exceptions for `Tcp.CompoundWrite` impl to build
Merge pull request #1750 from spray/backport-3581-io-compound-write

Backport of #3581 (addition of Tcp.CompoundWrite), some cleanup