Add keep-alive pings during payment

[roeierez]

Currently users get transport errors when payments takes more than a certain time (probably around 30 seconds).
It seems that it relates to the fact that the underline gRPC connection keep alive doesn't work as expected and that is under work in greenlight.
Meantime we can mitigate that by sending Ping requests over greenlight gRPC interface every let's say 15 seconds to keep the node connection alive.