Configure grpc keepalive settings

xds-relay should configure server keepalive settings. It will help the grpc server handle stale streams and connections from sidecars.
https://godoc.org/google.golang.org/grpc/keepalive#ServerParameters
```
type ServerParameters struct {
// MaxConnectionIdle is a duration for the amount of time after which an
// idle connection would be closed by sending a GoAway. Idleness duration is
// defined since the most recent time the number of outstanding RPCs became
// zero or the connection establishment.
MaxConnectionIdle time.Duration // The current default value is infinity.
// MaxConnectionAge is a duration for the maximum amount of time a
// connection may exist before it will be closed by sending a GoAway. A
// random jitter of +/-10% will be added to MaxConnectionAge to spread out
// connection storms.
MaxConnectionAge time.Duration // The current default value is infinity.
// MaxConnectionAgeGrace is an additive period after MaxConnectionAge after
// which the connection will be forcibly closed.
MaxConnectionAgeGrace time.Duration // The current default value is infinity.
// After a duration of this time if the server doesn't see any activity it
// pings the client to see if the transport is still alive.
// If set below 1s, a minimum value of 1s will be used instead.
Time time.Duration // The current default value is 2 hours.
// After having pinged for keepalive check, the server waits for a duration
// of Timeout and if no activity is seen even after that the connection is
// closed.
Timeout time.Duration // The current default value is 20 seconds.
}
```
MaxConnectionIdle : should be a few hours
MaxConnectionAge : default infinity works well
MaxConnectionAgeGrace: few minutes
Time: 2minutes
Timeout: default 20s works well