TCP keepalive

TCP keepalive has been enabled for the QuestDB socket connection to avoid disconnects caused by client inactivity.
Should work in most cases; if the connection goes through a firewall this solution might not be good enough.
In that case users could add empty messages on application layer probably to keep the connection alive.
The long term solution is definitely to improve the client further and implement keepalive on ILP protocol level (or whatever will be replacing it in the future).