Make metadata exception map lock-free

[adamatousek]

To date, the metadata exception map is implemented using an std::mutex-guarded std::map. Performance-wise, we would benefit from using a concurrent hash table.

[adamatousek]

Current plan is to implement it in a similar way as snapshots: mutable objects have metadata exceptions in a local map and once committed, the map is serialised and stored in a (shared) auxiliary pool.