DfsPackFile: Enable/disable object size index via DfsReaderOptions

DfsPackFile: Enable/disable object size index via DfsReaderOptions

DfsPackFile always uses the object size index if available. That is
the desired final state, but for a safe rollout, we should be able to
disable using the object size index.

Add an option (dfs.useObjectSizeIndex) to enable/disable the usage of
the object size index. False by default.

This changes the default from true to false. It only makes a different
for the DFS stack when writing of the index was explicitely
enabled. This is an optimization, so it shouldn't cause any
regression. Operators can restore previous behaviour setting
"dfs.useObjectSizeIndex" to true.

Change-Id: I44bf5a57e3942a4ecfe66d58bfa9175e99f96fcc