xAODRootAccess: thread-checker fixes

Various (hopefully uncontroversial) thread-checker fixes for xAODRootAccess. There are no functional changes in this MR. Just various const/mutables fixes and suppression of thread-checker warnings where applicable:

• suppress warning about unavoidable const_cast due to DataBucketBase
• make branch() non-const in TPrimitiveAuxBranchManager and TAuxBranchManager
• THolder: suppress warning from TClass::GetBaseClassOffset
• mark TFileAccessTracer as thread-safe (uses internal locks)
• mark const_cast in TEvent::setActive as thread-safe
• TAuxStore: mark usage of SG::AuxTypeRegistry as tread-safe and const fix
• Remove mutables in TAuxStore in favor of const-casting this where needed. This gives better control over potential thread-safety issues.
• mark MakeTransientTree as not thread-safe

cc @akraszna @ssnyder

Relates to ATEAM-863.