xAODRootAccess: thread-checker fixes

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.