kad: Investigate substream reuse and bound the number of open substreams

[lexnv]

Litep2p is currently creating a new substream for handling Kademlia commands.
Reutilizing a previously used substream might be beneficial from a performance standpoint.

While at it, bound the number of substreams a peer can open to a sensible number.

A few more details wrt libp2p implementation:

• reutilization of substreams on Waiting state: https://github.com/libp2p/rust-libp2p/blob/858a4cd954bf52be108ebc63bbb00b52964770dd/protocols/kad/src/handler.rs#L519-L541
• entering the Waiting state: https://github.com/libp2p/rust-libp2p/blob/858a4cd954bf52be108ebc63bbb00b52964770dd/protocols/kad/src/handler.rs#L918-L927

[dmitry-markin]

We should also take into account that substreams are cheaper than connections and it might be beneficial to keep the code simple if there are no indication we are hitting a bottleneck here.