Update locking management for iter_fwd and iter_hints methods. (#1054)

Update locking management for iter_fwd and iter_hints methods. (#1054)

fast reload, move most of the locking management to iter_fwd and
iter_hints methods. The caller still has the ability to handle its
own locking, if desired, for atomic operations on sets of different
structs.

Co-authored-by: Wouter Wijngaards <wcawijngaards@users.noreply.github.com>