Potential data race on acceccing `m_direntLookup`

[mgautierfr]

Investigating cache configuration, it seems that we have a potential data race about accessing/initializing m_direntLookup.

This data race is in FileImpl::direntLookup() method : https://github.com/openzim/libzim/blob/main/src/fileimpl.cpp#L277-L293

The field m_direntLookup (std::unique_ptr) is accessed from different threads and std::unique_ptr is not thread safe. (https://learn.microsoft.com/en-us/cpp/standard-library/thread-safety-in-the-cpp-standard-library?view=msvc-170)
The lock we have only avoid us to create the dirent lookup twice but it doesn't prevent us to read and write m_direntLookup in the same time.