FIX: Open file handles 'just in time' during s3 migration (#28806)

Previously we were opening the file handles, then putting them in a queue for upload. If that queue grows too large, we can hit a maximum open files limit.

This commit opens the file handle 'just in time', so the maximum number of open handles is equal to the upload concurrency (20).

lib/file_store/to_s3_migration.rb

@@ -211,7 +211,8 @@ module FileStore
         UPLOAD_CONCURRENCY.times.map do
           Thread.new do
             while obj = queue.pop
-              if s3.put_object(obj[:options])
+              opts_with_file = obj[:options].merge(body: File.open(obj[:path], "rb"))
+              if s3.put_object(opts_with_file)
                 putc "."
                 lock.synchronize { synced += 1 }
               else
@@ -237,7 +238,6 @@ module FileStore
 
         options = {
           acl: SiteSetting.s3_use_acls ? "public-read" : nil,
-          body: File.open(path, "rb"),
           bucket: bucket,
           content_type: MiniMime.lookup_by_filename(name)&.content_type,
           content_md5: content_md5,