Race condition in Onlinesign during key rollover

In onlinesign module, during key rollover, the code goes to the point where keyset shall be reloaded:

		free_zone_keys(mod->keyset);
		free(mod->keyset);
		ret = knotd_mod_dnssec_load_keyset(mod, true);

If at the same time, another thread is answering from the same zone and it gets to signing phase, it attempts read from freed memory.

The solution is:

• load keyset to temporary pointer and exchange the pointers atomically
• RCU on the allocated keysets