rpc: scanblocks, add "completed" flag to the result obj

To tell the user whether the process was aborted or not.

Plus, as the process can be aborted prior to the end range,
have also changed the "to_height" result value to return the
last scanned block instead of the end range block.

test/functional/rpc_scanblocks.py

@@ -48,6 +48,7 @@ class ScanblocksTest(BitcoinTestFramework):
         assert blockhash in out['relevant_blocks']
         assert_equal(height, out['to_height'])
         assert_equal(0, out['from_height'])
+        assert_equal(True, out['completed'])
 
         # mine another block
         blockhash_new = self.generate(node, 1)[0]