Merge pull request #3627 from status-im/seen-ttl

Align `seen_ttl` with attestation lifetime

specs/phase0/p2p-interface.md

@@ -245,7 +245,7 @@ The following gossipsub [parameters](https://github.com/libp2p/specs/blob/master
 - `fanout_ttl` (ttl for fanout maps for topics we are not subscribed to but have published to, seconds): 60
 - `mcache_len` (number of windows to retain full messages in cache for `IWANT` responses): 6
 - `mcache_gossip` (number of windows to gossip about): 3
-- `seen_ttl` (number of heartbeat intervals to retain message IDs): 550
+- `seen_ttl` (expiry time for cache of seen message ids, seconds): SECONDS_PER_SLOT * SLOTS_PER_EPOCH * 2
 
 *Note*: Gossipsub v1.1 introduces a number of
 [additional parameters](https://github.com/libp2p/specs/blob/master/pubsub/gossipsub/gossipsub-v1.1.md#overview-of-new-parameters)