Add TCP Keep-Alive option to TcpInput
Add TCP Keep-Alive option to TcpOutput
Use alternative type coercion spelling for KeepAlive

TcpInput will return error if connection is not of type TCPConn.
TcpOutput will print error, and then skip KeepAlive.

Also added OutputRunner to the TcpOutput struct to support error
logging.
Merge pull request #888 from carlosdp/tcp-keepalive

Add TCP Keep Alive options to TcpInput/TcpOutput