config+lnd: allow configuring gRPC keepalive settings

This change allows users to customize the gRPC keepalive settings. The
server ping values configure when the server sends out a ping by itself
and how quickly the client is expected to respond.
The client ping min wait configures the minimum time a client has to
wait before sending another ping. A connection might be closed by the
server if a client pings more frequently than the allowed min wait time.

We choose lower default values than the gRPC library used before:
 - ServerPingTime: 1 minute instead of 2 hours
 - ClientPingMinWait: 5 seconds instead of 5 minutes

This should by itself already make long-standing gRPC streams more
stable but also allow clients to set a custom client ping time of 5
seconds or more (a value of slightly more than 5000 milliseconds should
be chosen to avoid the server disconnecting due to slight timing skews).

lnd.go

@@ -37,6 +37,7 @@ import (
 	"github.com/lightningnetwork/lnd/watchtower"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/credentials"
+	"google.golang.org/grpc/keepalive"
 	"gopkg.in/macaroon-bakery.v2/bakery"
 	"gopkg.in/macaroon.v2"
 )
@@ -303,10 +304,23 @@ func Main(cfg *Config, lisCfg ListenerCfg, implCfg *ImplementationCfg,
 		}
 	}()
 
+	// Allow the user to overwrite some defaults of the gRPC library related
+	// to connection keepalive (server side and client side pings).
+	serverKeepalive := keepalive.ServerParameters{
+		Time:    cfg.GRPC.ServerPingTime,
+		Timeout: cfg.GRPC.ServerPingTimeout,
+	}
+	clientKeepalive := keepalive.EnforcementPolicy{
+		MinTime:             cfg.GRPC.ClientPingMinWait,
+		PermitWithoutStream: cfg.GRPC.ClientAllowPingWithoutStream,
+	}
+
 	rpcServerOpts := interceptorChain.CreateServerOpts()
 	serverOpts = append(serverOpts, rpcServerOpts...)
 	serverOpts = append(
 		serverOpts, grpc.MaxRecvMsgSize(lnrpc.MaxGrpcMsgSize),
+		grpc.KeepaliveParams(serverKeepalive),
+		grpc.KeepaliveEnforcementPolicy(clientKeepalive),
 	)
 
 	grpcServer := grpc.NewServer(serverOpts...)