fix some warts in IO

- remove TcpPipelineHandler’s type parameter
- remove not-accept() failure test (which is completely bogus)
- add comments explaining more about TcpPipelineHandler.Init
- remove the funny “manager” from the Server example
add BackpressureBuffer, see #3253

- also make a Write’s “ack” be a Tcp.Event (to suit pipelines)
- add stress test for BackpressureBuffer
- add it to SslTlsSupportSpec
- add it to the docs
add ScalaDocs and correct UDP docs, see 3268
split IO docs and mark them EXPERIMENTAL, see #3294
fix one more warning
a few fixes after review of the IO doc changes
Merge pull request #1483 from akka/wip-IO-docs-∂π

several things IO