[sgen-parallel-collector] Replace worker stealable stack with section gray queue

This approach presents two main advantages :
 - remove copying of data around memory with memcpy, it is replaced by dequeuing/enqueuing sections which does not imply copying large amounts of memory, but only passing pointers
 - greatly simplify the code because we do not allocate section which then does not recursively call back workers_gray_queue_share_redirect