Increase Keep-Alive timeout

[utsavbshah]

I am using the gatsby serve command to serve my gatsby site.

When Gatsby is behind an AWS ELB/ALB, it can cause occasional 502 errors as the ELB's timeout(60s) is longer than Gatsby's. To prevent this it should be the other way around where Gatsby has a longer timeout than the ELB.

Ability to set in the Gatsby config a way to set the values for keepAliveTimeout.