Allow span processor to keep reference of spans

[TommyCpp]

The spec noted that we should allow span processor to keep a reference to the span passed as the parameter of on_start method

It SHOULD be possible to keep a reference to this span object and updates to the span SHOULD be reflected in it

[TommyCpp]

Like Cow, what if we have a enum of Span where it can contains a mutex if one of the span processor decided to keep a reference but if no span processor hold the reference, we can just have a span without mutex?