Parallelize sensor search

[Flix6x]

Tech spike to increase chart loading speed through parallelization. I'm using n=4 parallel processes here, but that could become a config setting.

@Ahmad-Wahid could you do a speed benchmark on this branch (compared to main) for me? Some manual tests of mine (locally) suggested an asset chart with 4 sensors and 1 week of data loads about twice as fast. I'm interested to see whether you can reproduce that result on a cloud server, and how the speed-up factor would change with a larger time frame and with more sensors.