io tests: clean up imports
io: change IOException treatment from "fatal error" to "expected during normal operation"

Up to now IOExceptions during reading, writing or connecting were treated as
fatal actor errors, crashing the connection actor and thus producing ERROR
level log messages. This patch treats such exceptions as "expected" during
normal operation and prevents them from crashing the actor. Rather, they are
logged at DEBUG level and the actor is actively and cleanly stopped.
Merge pull request #1536 from spray/master

io: tone down error logging a bit more