Fix 3 dead links to a paper in akka-docs.

Make them point to an Akka mirror of the paper instead.
Merge pull request #1402 from CodeBlock/deadlinks

Fix 3 dead links to a paper. (More docs fixes)
Some more test cleanup to not create so many threads. See #3320
Merge pull request #1410 from akka/wip-3320-to-many-threads-again-ban

Some more test cleanup to not create so many threads #3320
io-tests: simplify TcpConnectionSpec in preparation for coming modifications

This doesn't change any test logic, it merely structures the test fixture code a bit differently to make it easier to insert additional mocks required for coming additions.
io: small clean-ups
io: remove SelectionKey lookup and reduce IO-layer-internal message volume
io: add comments based on review feedback