deadlock when add duplicated labels by multithreads

[cdjingit]

Hello, everyone,

Some labels are duplicate in our case. We add labels by multi-thread.
I found thread may lock the same mutex two times in one addPoint which lead to deadlock.
The first time to lock mutex at here: https://github.com/nmslib/hnswlib/blob/master/hnswlib/hnswalg.h#L1021
The second time to lock the same mutex at here: https://github.com/nmslib/hnswlib/blob/master/hnswlib/hnswalg.h#L185

[dyashuni]

Hi @cdjingit!
Thank you for reporting it. Do you have an example to reproduce it?

[dyashuni]

@cdjingit I reproduced the issue. I will take a look at this.