s3: route single-entry object writes to the owner filer, off the DLM

For non-versioned, non-object-lock buckets, PutObject and unversioned
DeleteObject now send their metadata write straight to the object key's owner
filer (resolved from the lock-ring view) with the precondition attached. The
owner serializes the write with its local per-path lock and evaluates the
condition atomically, so these paths no longer acquire a distributed lock.

The fast path is opt-in per request and falls back to withObjectWriteLock for
anything it does not fully cover: versioned or object-lock buckets, conditions
that do not reduce to a single primitive (ETag lists, weak ETags, time-based,
combined headers), an unresolved owner, or an unreachable owner filer. So
behavior is unchanged outside the safe subset.

Multi-step finalizations (copy, CompleteMultipartUpload) and versioned writes
keep the distributed lock: they mutate several entries under one held lock,
which a single conditional create or delete does not cover.

DeleteEntry gains the same optional WriteCondition and per-path lock as
CreateEntry so a routed conditional delete is atomic on the owner.

other/java/client/src/main/proto/filer.proto

@@ -304,11 +304,16 @@ message DeleteEntryRequest {
     bool is_from_other_cluster = 7;
     repeated int32 signatures = 8;
     int64 if_not_modified_after = 9;
+    // Optional precondition evaluated against the current entry atomically with
+    // the delete, under the filer's per-path lock. Only honored for a
+    // non-recursive single-entry delete routed to the entry's owner filer.
+    WriteCondition condition = 10;
 }
 
 message DeleteEntryResponse {
     string error = 1;
     SubscribeMetadataResponse metadata_event = 2;
+    FilerError error_code = 3; // machine-readable error code (e.g. PRECONDITION_FAILED)
 }
 
 message AtomicRenameEntryRequest {