VersionSet::RemoveFileLevelBloomFilterInfo isn't thread-safe about pebblesdb

[bx1n]

@cryptokat I was trying to reproduce the bug to get a better understanding of the bug. Can you explain how you were able to produce this error?

[erangi]

I've hit this issue too when running a multi-threaded benchmark (using YCSB-C). The std::map isn't thread-safe, and gets corrupted by multiple concurrent write accesses. This is manifested by either this crash or an infinite loop during map traversal. The solution is probably to mutually exclude accesses to file_level_bloom_filter. The lowest overhead way is using a read-write lock (e.g., std::shared_mutex), but that requires C++14/17. Using std::mutex will have a somewhat higher overhead, but at least allow correct concurrent runs.