[netgroupman] Move asmap checksum calculation to NetGroupManager

src/netgroup.h

@@ -6,6 +6,7 @@
 #define BITCOIN_NETGROUP_H
 
 #include <netaddress.h>
+#include <uint256.h>
 
 #include <vector>
 
@@ -22,6 +23,9 @@ public:
      * exists, since the data is const. */
     const std::vector<bool>& GetAsmap() const { return m_asmap; }
 
+    /** Get a checksum identifying the asmap being used. */
+    uint256 GetAsmapChecksum() const;
+
     /**
      * Get the canonical identifier of the network group for address.
      *