Make CBlock.get_merkle_root static

We'll later need this method to calculate merkle roots outside of CBlock.
I'd like to avoid moving this code outside of CBlock as it might later
conflict with Bitcoin backports.

qa/rpc-tests/test_framework/mininode.py

@@ -549,7 +549,8 @@ def serialize(self):
         return r
 
     # Calculate the merkle root given a vector of transaction hashes
-    def get_merkle_root(self, hashes):
+    @staticmethod
+    def get_merkle_root(hashes):
         while len(hashes) > 1:
             newhashes = []
             for i in range(0, len(hashes), 2):