Increase RPC max RT time to 7s

[bcherry]

There are some cases where we see ~6s delays. We are separately working to track those down and improve the worst-case speed. The DC reliability work from the summer has addressed the vast majority of true lost packets so it's better to keep waiting than to fail early.

Three changes

• Increase max RT to 7s (it needs to be at least 6s, this felt like a conservative choice for now)
• Increase default response timeout by 15s (keeps the effective handler time to 8s as it was already)
• Add missing response timeout adjustment to account for RT time (other SDKs had this already)

[xianshijing-lk]

Two quick questions. lgtm

[xianshijing-lk]

thanks, lgtm.