Detect when SendingConnections disconnect even if we aren't sending to them

This patch modifies SendingConnection to always select for reading; when the conneciton is readable, it reads a byte to detect EOF or an I/O Error and trigger connection shutdown, allowing sent but unack'd messages to fail instead of hanging.

This also adds a test to DistributedSuite which can trigger the issue by doing memory-to-memory replications operations around failures.

This addresses issue SPARK-704.