[Feature] Refactor Lazy SMP to take in different single process search functions

[KYLChiu]

          As discussed offline PVS can also be parallelized but given we don't use LAZY_SMP at all for now, for a future task.

Originally posted by @KYLChiu in #141 (comment)

If we can refactor LAZY_SMP to take different search functions (PVS, negamax_sp), that would be great. All searcher classes should have _start_search_from_root somewhere, which can be called.

#141 - PVS is in process but the skeleton is here.