Fix function-local static to thread_local
Give G4cout to HepMC::GenEvent::print as a thread-safe ostream
Explicitly disable support for SimWatchers and external SimActivityRegistry when running with more than 1 thread

SimWatchers are in general thread-unsafe (although there are some that
are thread-safe), and the situation with external SimActivityRegistry
is unclear. It is assumed that the use cases have single-threaded
nature anyway.
Add CustomUIsessionThreadPrefix and CustomUIsessionToFile to ease MT debugging
Merge pull request #6264 from makortel/geant4mtfixes

Disable SimWatchers for MT sim in presence of more than one thread + additional fixes