Use LastSeen as libp2p seen messages cache strategy

By default, the libp2p seen messages cache uses the `FirstSeen` strategy
which expires an entry once TTL elapses from the time it was added.
This means that if a single message is being received frequently and
consistently, pubsub will re-broadcast it every TTL, rather than never
re-broadcasting it.

In the context of the Keep client which additionally uses app-level
retransmissions, that often leads to a strong message amplification in the
broadcast channel which cause a significant increase in the network load.

As the problem is quite common
(see libp2p/go-libp2p-pubsub#502), the libp2p team
added a new `LastSeen` strategy which behaves differently. This strategy
expires an entry once TTL elapses from the last time the message was touched
by a cache write (`Add`) or read (`Has`) operation. That gives the desired
behavior of never re-broadcasting a message that was already seen within the
last TTL period. This reduces the risk of unintended over-amplification.

pkg/net/libp2p/channel_manager.go

@@ -9,6 +9,7 @@ import (
 	"github.com/keep-network/keep-core/pkg/net"
 	"github.com/keep-network/keep-core/pkg/net/retransmission"
 	pubsub "github.com/libp2p/go-libp2p-pubsub"
+	pubsubtc "github.com/libp2p/go-libp2p-pubsub/timecache"
 	"github.com/libp2p/go-libp2p/core/host"
 	"github.com/libp2p/go-libp2p/core/peerstore"
 )
@@ -51,6 +52,7 @@ func newChannelManager(
 		pubsub.WithMessageSignaturePolicy(pubsub.StrictSign),
 		pubsub.WithPeerOutboundQueueSize(libp2pPeerOutboundQueueSize),
 		pubsub.WithValidateQueueSize(libp2pValidationQueueSize),
+		pubsub.WithSeenMessagesStrategy(pubsubtc.Strategy_LastSeen),
 	)
 	if err != nil {
 		return nil, err