[C++] Make Headers multi-thread safe

Currently headers are not multi-thread-safe, and the calling application needs to take care on having the right multithread-behaviour.

A simple optional (?) option would be to introduce a header-mutex with a lockguard on each object. This introduces a memory overhead, but would make concurrent calls possible if they are on separate objects.

No files or tests are named. Start by locating the generated C++ headers and reviewing how objects are accessed concurrently; the issue does not define whether locking is required, optional, or what completion criteria should be used, so the synchronization design needs clarification first.

Written by the indexing model from the issue text.