fix: decrease default routing table size

[achingbrain]

Reduces the default routing table size to 1280 (down from 5120).

This improves CPU/memory/network usage as we don't need to sort as many peers when finding the closest ones to a given key, nor do we need to contact as many to ensure only live peers are still in the table.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works

[2color]

How does this compare to the defaults in https://github.com/libp2p/go-libp2p-kad-dht?

[achingbrain]

How does this compare to the defaults in libp2p/go-libp2p-kad-dht?

go-libp2p-kad-dht uses a different data structure for it's routing table, something more similar to #2747 so a direct comparison is difficult.

Instead js-libp2p has a binary trie with peers equally distributed across the address space - the idea is this results in faster lookups for arbitrary data since we know of more peers further from our own peer id. To ensure we stay well connected it also maintains a separate list of the 20 peers closest to the node's peer ID so far encountered.