Use search_pool to control iterator->Next()

[alwayslove2013]

The current AnnIterator function utilizes the search pool for concurrency control only when initializing the Iterator. Once the returned iterator is handed over to the upper layer, the ->next() calls are not subjected to any thread restrictions, which may lead to issues such as OMP conflicts.

To address this, we propose the following considerations:

• All iterators will accept a use_knowhere_search_pool parameter during construction.
  • When set to True (the default), the iterator->next() will be scheduled by the knowhere_search_thread_pool.
  • When set to False, iterator->next() will not involve thread scheduling internally, so please take caution.
• The initialization of iterator in the AnnIterator function will no longer be concurrent, which helps to avoid potential deadlocks.
  • Furthermore, to enhance performance for large_nq, we will try to streamline the initialization process for all iterators by deferring heavier pre-computation tasks to the first call of ->next().