Redundant load in LOS per-object metadata initialization

[wenyuzhao]

Looks like this metadata load https://github.com/mmtk/mmtk-core/blob/master/src/policy/largeobjectspace.rs#L54-L59 is redundant?

The following store_metadata call will perform "load - mutate bits - store" internally.

[qinsoon]

Is this for side metadata or header metadata or both? I guess we can introduce a mutate_metadata() that takes a closure for how to mutate the bits.

[wks]

This may probably be made redundant by #847