messaging: enable keepalive tcp option for inter-node communication
messaging: enable keepalive tcp option for inter-node communication

Some network equipment that does TCP session tracking tend to drop TCP
sessions after a period of inactivity. Use keepalive mechanism to
prevent this from happening for our inter-node communication.

Message-Id: <f9ad7188f51e169095538a44310227ed1d2d4269@scylladb.com>

raphaelsc/scylla,duarten/scylla,raphaelsc/scylla,kjniemi/scylla,duarten/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,scylladb/scylla,scylladb/scylla,kjniemi/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla