Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/14749
Add `--num_dbs` flag to run N independent DB instances in parallel. Each `StressTest` instance has its own DB with isolated fault injection (from D104959945). `db_crashtest.py` defaults to `num_dbs=1`.
For `num_dbs=1`: `--db` and `--expected_values_dir` are paths used as-is.
For `num_dbs>1`: they are parent directories; C++ creates `db_0/`, `db_1/`, ... subdirs underneath.
Path ownership: C++ owns DB and secondary dir creation (supports remote env). Python owns EV dir creation (always local). C++ also creates EV dirs as fallback for direct CLI usage. `DestroyAllDbs` cleans up subdirs and the parent dir.
Per-DB: `threads`, `max_key`, `ops_per_thread`, `reopen`, `column_families`, and all DB options.
Shared: background env threads (compaction, flush pool), `block_cache`, `write_buffer_manager`, `compressed_secondary_cache`, `rate_limiter`, `compaction_thread_pool_adjust_interval`.
Reviewed By: anand1976
Differential Revision: D104959942
fbshipit-source-id: 3d0d60101e7f2e600306e5a9c4018686bf649658
c724aeb67e