rust volume: group-commit fsync uploads on the direct write path (#11860)

* rust volume: group-commit fsync uploads on the direct write path

Each fsync=true upload used to run its own .dat/.idx sync when the write
queue was off, which is the default. Route durable uploads through the
per-volume write workers so a batch shares one sync, matching Go's
writeNeedle2: non-fsync writes, and writes while the server is stopping,
stay on the synchronous path. The queue is now always constructed and
SEAWEED_WRITE_QUEUE is gone.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* rust volume: document post_handler write routing

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

seaweed-volume/src/server/profiling.rs

@@ -170,7 +170,6 @@ mod tests {
             grpc_allowed_wildcard_domain: String::new(),
             grpc_volume_allowed_common_names: vec![],
             tls_policy: TlsPolicy::default(),
-            enable_write_queue: false,
             security_file: String::new(),
         }
     }