Race condition in OnHeapHnswGraph

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Hi all,

I'm looking at `OnHeapHnswGraph` code and noticed that the volatile field
`graphRamBytesUsed` is modified in `addNode` in a racy way:

  long bytesUsed = graphRamBytesUsed;
  graphRamBytesUsed = bytesUsed + l;

This is equivalent to `graphRamBytesUsed += l`.

This code is susceptible to lost update due to non-atomic read-modify-write
operation.

I guess the it's not really a problem, because this code is in fact
single-threaded when documents are added to the index. It might be
concurrent during merging, but then `ramBytesUsed()` isn't called, and it's
a wasted work.

If the above assumption is correct, then this field should not be volatile
to improve performance. If it's not, it should be replaced with
`AtomicLong` or `LongAdder`.

Viliam

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