tcp connection actors, see #2886
tcp selector and listener actors, extension of tcp manager actor, see #2885 and #2887
refactor tests to reuse common connection setup
fix assertion error typo
fix issues discussed in the pull request
use a direct buffer pool for buffers needed for channel.read/write

The major advantage of this approach is that a Write is only copied once
into its direct buffer and this direct buffer is kept until it is written
fully.
in tests make sure to get a server address that is highly probable to be currently unbound
improvements from the first round of feedback, see #2885 and #2887
more tests, smaller improvements
improve DirectByteBufferPool and some smaller fixes

 * dispatch Closed events to the sender of the close command as well
 * DirectByteBufferPool changes:

     * hide behind BufferPool interface
     * only return buffers of the configured size
     * reduce work to be done while locked
     * use Array-based stack to store free buffers instead of linked lists
     * keep buffers by soft reference
remove commented code
Revert "in DirectByteBufferPool keep buffers by soft reference"
make asserting on received messages more resilient
improve stability of tests, pullFromServerSide now uses real selector to check for conditions
further improve pullFromServerSide stability
add more integration tests
fix race condition when trying to assert closed channel on the server side
use proper selection strategy to try to improve test stability
properly cleanup selector in tests
Change socket options type from `immutable.Traversable` to `Traversable`

The problem with `immutable.Traversable` is that it doesn't allow the use of `Seq.apply` for socket options, since `Seq` is aliased to `scala.collection.Seq` and not `scala.collection.immutable.Seq` in `package object scala`. Even though technically nicer `immutable.Traversable` therefore hinders usability of the API, for a benefit that we don't consider worth the cost.

Conflicts:
	akka-io/src/main/scala/akka/io/TcpConnection.scala