Use dashmap+fxhash instead of flurry (#2431)
While profiling, I noticed we spent a lot of time locked in `get_or_create_persistent_task` while flurry's HashMap was resizing itself[1]. I swapped it with dashmap and I'm seeing 8% improvements on our 10k modules benchmark.
<img width="813" alt="image" src="https://user-images.githubusercontent.com/1621758/198401787-410eb6b6-e850-4003-a34d-6077c6d264e8.png">
<img width="689" alt="image" src="https://user-images.githubusercontent.com/1621758/198402178-0b34d412-d921-456a-8c6a-86f255952694.png">
Co-authored-by: Tobias Koppers <1365881+sokra@users.noreply.github.com>
