(ITS#8849) Notify backends of server (un)pause

Full_Name: Ondrej Kuznik Version: master OS: URL: https://github.com/mistotebe/openldap/tree/ITS8849 Submission from: (NULL) (82.10.24.68)

If a backend needs to handle its own long-running threads on the side, it might need to be notified to pause while a config modification is underway and when it is finished.

This is the case of the load balancer module, which registers itself as a module and maintains its own threads and needs to know when a server (or even its own) reconfiguration is in progress.

The linked patches provide one of the possible implementations, they introduce the hooks to pause and unpause the server, use them in bconfig.c and add two new backend hooks that are consulted while this is happening.