Implement Gossip seen_ttl parameter

[Nashatyrev]

Currently the seen message buffer is an LRU Set with hardcoded length 10000
We need more flexible configuration on this structure (ethereum/consensus-specs#1958):

• seen_ttl: how many heartbeats the message ID shouldn't be pruned from the buffer
• seen_max: should limit the buffer size (to avoid out of mem) and in case of overflow delete older entries overriding previous parameter behavior