Large clusters with multiple sets, or multi-pool setups at times might
fail and report unexpected "file not found" errors. This can become
a problem during startup sequence when some files need to be created
at multiple locations.

- This PR ensures that we nil the erasure writers such that they
  are skipped in RenameData() call.

- RenameData() doesn't need to "Access()" calls for `.minio.sys`
  folders they always exist.

- Make sure PutObject() never returns ObjectNotFound{} for any
  errors, make sure it always returns "WriteQuorum" when renameData()
  fails with ObjectNotFound{}. Return appropriate errors for all
  other cases.