SERVER: Receving SIGSEGV process on shutdown
    
    There is race condition when dynamic libraries are unloaded. Talloc
    library calls our destructors but they still need openssl calls
    which might be not available.
    
    Solution is to free explicitely memory context and trigger
    destructors before calling exit(). In this PR the SIGTERM
    handler is moved from individual providers to generel
    backend code.
    
    Also generic server code is changed to explicitely free
    memory context when signal is received.
    
    Resolves:
    https://bugzilla.redhat.com/show_bug.cgi?id=1672584
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Sumit Bose <sbose@redhat.com>