Main/Medium+AutoCaller+others: fix medium uninit deadlock caused by lock order violations, sometimes taking the caller before the media tree lock, sometimes after, plus a few other small fixes

Main: warning

Main: AutoCaller/VirtualBoxBase refactoring, cleanly splitting out the object state handling, and moving all caller synchronization to one file. Also eliminated a not so vital template (AutoCallerBase) by much simpler inheritance. Theoretically has no visible effects, the behavior should be identical. Done as a preparation for reimplementing the caller synchronization.