Mostly "cosmetic" thread-checker fixes as most of this code actually
never runs multi-threaded:
- `xAOD::Init`: use `atomic_flag` to serialise `Init` and various const
  fixes and warning suppressions. Also fix a bug where the return value
  of `SetErrorHandler` was used to judge if the error handler was installed.
  But the return value of that function is the previous signal handler.
  So there is no need to even store that value.
- Mark `TFileChecker` as not thread-safe.
- Remove unnecessary mutable in `TAuxVectorFactory.
- Mark `THolderCache` and `TDestructorRegistry` singletons as thread-safe as
  all methods use mutexes.
- Simplify singleton creation in `TEventFormatRegistry`.
- Return `const` pointer in `TEvent::getBranchInfo`.