set TCP keep alive

Change-Id: I4ed76f1230a84a8bd19ea8503d79be7bdcc9ddc0
Reviewed-on: http://review.couchbase.org/41452
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: John Liang <johliang00@gmail.com>