enable TCP_KEEPALIVE to tcp connections #1751

Background

In the context of the asymmetric recent disconnection issues #1206, enabling TCP_KEEPALIVE could help dropping connections as soon as they reach a zombie state. This is done bidirectionally at the OS level, so it is orthogonal to the ping/pong in #1687

User Story

No response

Acceptance Criteria

The TCP connections are configured to use an appropriate TCP_KEEPALIVE setting

Resources & Additional Notes

This is a natural follow-up to #1712