scripted-diff: rename m_cs_callbacks_pending -> m_callbacks_mutex

refactor: replace RecursiveMutex m_callbacks_mutex with Mutex

In each of the critical sections, only the the guarded variables are
accessed, without any chance that within one section another one is
called. Hence, we can use an ordinary Mutex instead of RecursiveMutex.

This is a backport of core#24069

with clang and debug
ninja && ninja check check-functional