snapshot: Improve memory utilization of snapshot fetcher (#2715)

Signed-off-by: Kim Altintop <kim@eagain.io>
Co-authored-by: Mazdak Farrokhzad <twingoow@gmail.com>

Author: kim

Cargo.lock

@@ -17,9 +17,11 @@ spacetimedb-fs-utils.workspace = true
 
 blake3.workspace = true
 bytes.workspace = true
+crossbeam-queue.workspace = true
 futures.workspace = true
 hex.workspace = true
 log.workspace = true
+scopeguard.workspace = true
 tempfile.workspace = true
 thiserror.workspace = true
 tokio = { workspace = true, features = ["io-util"] }