Yuki Kishimoto on Nostr: Unlike the REQs, the negentropy messages can take only one filter, so it may be ...

Unlike the REQs, the negentropy messages can take only one filter, so it may be useful in some case to allow multiple sync at the same time. But it's also true that sync can be executed in sequence, like I'm doing with `Relay::sync_multi`: https://github.com/rust-nostr/nostr/blob/e4dbf484a8d1e57a05f0cc0312dd52c6c43cddd5/crates/nostr-relay-pool/src/relay/inner.rs#L1776

Here I use the `sync_multi`: https://github.com/rust-nostr/nostr/blob/e4dbf484a8d1e57a05f0cc0312dd52c6c43cddd5/crates/nostr-sdk/src/client/mod.rs#L1850