StorageLoader: add tcpKeepAlive=true to JDBC for long-running COPYs via NAT

[alexanderdean]

Otherwise we get the dread:

An I/O error occured while sending to the backend

For people who find this in the future: don't forget to configure your client's kernel to use TCP keep-alive correctly too:

• http://tldp.org/HOWTO/TCP-Keepalive-HOWTO/usingkeepalive.html
• http://docs.aws.amazon.com/redshift/latest/mgmt/connecting-firewall-guidance.html

JDBC settings: https://jdbc.postgresql.org/documentation/head/connect.html

[alexanderdean]

Update: I'm not sure the TCP keep-alive settings in the kernel make a material difference...