xAODRootAccess: use thread-safe TSMethodCall and add locks

Another step in making this package thread-safe:

• Use the thread-safe TSMethodCall instead of TMethodCall in TAuxVectorFactory.
• Add locks for the local static missingSGKeys in TEventProxyDict.
• Add upgrade_mutex for the BranchInfo cache in TEvent.

Relates to ATEAM-863 and requires !59031 (merged).

cc @akraszna