Fix too many heap sorts to improve getNeighborsByHeuristic2

[shengjun1985]

Fix too many heap sorts to improve getNeighborsByHeuristic2

Signed-off-by: shengjun.li shengjun.li@zilliz.com

[shengjun1985]

@yurymalkov
Would you like to review this PR?
I didn't change any calculation logic. I just changed std::vector<std::pair<dist_t, tableint>> return_list to std::vector<tableint> return_list to reduce the times of heap sorting.

Here is the addPoint() performance comparison on my local machine.
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz * 8
dataset: sift_1M (row count = 1000000, dim = 128, space = L2)

	0.5.0	PR-298
M = 16, ef_construction = 200	522411ms	499639ms
M = 16, ef_construction = 100	274406ms	265631ms

[yurymalkov]

@shengjun1985 It somehow slipped through my attention, sorry. I am going to review it within a few days.
@dyashuni also please review if you have time.