User iterator prefetching in hashtree module

Use prefetch and prefetch_stop actions for iterator_move calls in the hashtree
module. This replaces the use of the next action with the goal of making
segment traversal more efficient. This change adds a prefetch field to the
itr_state record and some additional case clauses in the iterate/2 function.
This ensures that an iterator can be safely used with other actions after
being used for prefetch iteration.