Changed statics to thread_local in EgammaTowerIsolation

To quickly fix the thread safety problems, the static data members
of EgammaTowerIsolation have been changed to be thread_local.
This avoids disturbing the present behavior and client APIs while
fixing the threading problems crashing jobs.
However, this should not be considered the permanent solution. Instead
the statics should become regular members and client code should
stop making temporary EgammaTowerIsolation objects and instead
keep reusing the one they make.