Reduce number of zbx_handle_log() calls in preprocessing

zbx_handle_log() function is called in process main loops to ensure proper stdin/stderr redirection after log has been rotated. However when process is not sleeping this adds unnecessary overhead, especially noticeable in pre-processing manager.

The zbx_handle_log() calls in preprocessing manager must be throttled to be called once per second. We might consider even adding now parameter to zbx_handle_log() and do throttling for all processes (though it probably has low % performance impact in sleep based main loops).