introduce packetqueue::unlock to unlock packets and notify anyone waiting. Also check for iterators pointing to packets when not keeping keyframes. Loop with waiting when we can't lock a packet

src/zm_packetqueue.h

@@ -84,6 +84,7 @@ class PacketQueue {
         unsigned int pre_event_count
     );
     bool is_there_an_iterator_pointing_to_packet(ZMPacket *zm_packet);
+    void unlock(ZMLockedPacket *lp);
 };
 
 #endif /* ZM_PACKETQUEUE_H */