fix checking for connection close in TcpConnectionSpec

The fix was to only rely on actually selecting not also on checking
key.readyOps since that isn't necessarily reliable without selecting.
Merge pull request #1066 from jrudolph/wip-IO-fix-close-race-condition

fix checking for connection close in TcpConnectionSpec
aggregate received data as long as there's data in kernel buffers or 'received-message-size-limit' is reached, see #2886
fix: skip selector when read is split up just because of ReceivedMessageSizeLimit