s3: route metadata-only self-copy off the distributed lock (#9638)

A non-versioned metadata-only self-copy (CopyObject with source == destination
and the REPLACE directive) is a read-modify-write of one entry, which is why it
held the distributed lock. It now routes to the owner as a serialized
PATCH_EXTENDED: the owner merges the new managed metadata (set the replacements,
delete the dropped keys) onto a fresh read of the entry under its per-path lock,
so a concurrent change to non-managed keys (legal hold, retention, version id) is
preserved instead of clobbered, and bumps mtime.

PATCH_EXTENDED gains touch_mtime for the mtime bump. Versioned and suspended
self-copies create a new version (already routed via the copy finalize) and the
no-owner bootstrap keep the lock.

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

@@ -316,6 +316,7 @@ message ObjectMutation {
     Recompute recompute = 9;               // RECOMPUTE_LATEST parameters
     bool set_content = 10;                 // PATCH_EXTENDED: replace Entry.content with content
     bytes content = 11;                    // PATCH_EXTENDED: new Entry.content when set_content
+    bool touch_mtime = 12;                 // PATCH_EXTENDED: set the entry's Mtime to now (e.g. a metadata-replace copy)
 }
 
 // Recompute re-derives a pointer entry (directory/name on the mutation) from the