Before this change, serve s3 did not consistently save the correct modtime value
in memory after putting or copying an object, which could sometimes cause an
incorrect modtime to be returned. This change fixes the issue by ensuring that
both "mtime" and "X-Amz-Meta-Mtime" are updated in b.meta when we have fresh data.
The issue was discovered on the TestBisyncRemoteRemote/ext_paths test.
nielashandNick Craig-Wood
c13118246c