Add mutexes to state for the queue and the allocator

Our threads need to negotiate access and mutation to the two
resources - we don't want them treading on each others toes when
generating new child nodes /or/ when trying to remove/add work to the
queue.  The former situation is particularly worrisome due to how weak
the indexing system is with the allocator.

src/state.hpp

@@ -8,6 +8,7 @@
 #ifndef STATE_HPP
 #define STATE_HPP
 
+#include <mutex>
 #include <queue>
 
 #include "base.hpp"
@@ -20,6 +21,9 @@ namespace cw::state
     cw::node::NodeAllocator allocator;
     std::queue<u64> queue;
 
+    std::mutex mutex_queue;
+    std::mutex mutex_allocator;
+
     State(void);
   };