fix: start with bucket idx 0 if local key

[mattsse]

If the local_key is provided for ClosestBucketsIter start it start in Start(BucketIndex(0)) instead of Done. otherwise, the closest_* iterators will always be empty and make it impossible lookup the local node in the DHT.

this is also the state libp2p-kad starts with if the local key was provided.

https://github.com/libp2p/rust-libp2p/blob/e530118fb017122786180570b2b68f89aac53905/protocols/kad/src/kbucket.rs#L361-L365

See also: https://docs.rs/libp2p-kad/0.41.0/src/libp2p_kad/behaviour.rs.html#831-856

[AgeManning]

Yeah nice, thanks.

I've been meaning to add the standard kad bootstrap query as a general function in the discv5 struct, just never got around to it, and hence why this bug has been left unnoticed.

Cheers :D