Enable max_background_jobs

[xiaobiaozhao]

Use max_background_jobs instead of max_background_compactions and max_background_flushes

https://github.com/facebook/rocksdb/blob/main/include/rocksdb/options.h#L699

[torwig]

Since max_background_compactions and max_background_flushes are deprecated, I think it's a good idea to replace them with max_background_jobs.