Creates an empty cache.
capacity is how many recently seen packets to remember, defaulting to
[MESH_SEEN_DEFAULT_CAPACITY]. A capacity of zero remembers nothing, so
every copy of a packet is relayed.
Optionalcapacity: number | nullReports whether a packet is currently remembered, without recording it.
Records a packet and reports whether it was new.
A true answer is when a node should act on the packet and relay it; a
false one means another copy already arrived by a different path.
A memory of recently seen packets, so a node relays each one only once.