Remove the shared state and the mutex from NVTX internals

[achirkin]

Until now, raft has stored a map of NVTX colors (annotation -> color) to avoid using the same color for different annotations and keep using the same color for the same annotations. This map is a shared state.
During an extensive ANN_BENCH throughput testing it has turned out that the mutex guarding the map can sometimes become a bottleneck when the number of concurrent threads is really large (>~ 256). This PR replaces the unordered map and the mutex guarding it with a deterministic hash value of the annotation instead (which is stateless).

Pros:

• No shared state, no mutexes.
• Assigns the same colors to the same annotations across program runs.

Cons:

• Sometimes different annotations can have the same color (hash collisions).

[tfeher]

Thanks Artem for the PR, looks good, just a small update in docstring is missing!

[tfeher]

Thanks for the update, LGTM!

[achirkin]

/merge