rpc: various fixups for dumptxoutset

- Actually generate an assumeutxo hash and display it
- Add nchaintx to output (necessary for use in chainparams entry)
- Add path of serialized UTXO file to output

test/functional/rpc_dumptxoutset.py

@@ -45,6 +45,10 @@ class DumptxoutsetTest(BitcoinTestFramework):
             assert_equal(
                 digest, '7ae82c986fa5445678d2a21453bb1c86d39e47af13da137640c2b1cf8093691c')
 
+        assert_equal(
+            out['txoutset_hash'], 'd4b614f476b99a6e569973bf1c0120d88b1a168076f8ce25691fb41dd1cef149')
+        assert_equal(out['nchaintx'], 101)
+
         # Specifying a path to an existing file will fail.
         assert_raises_rpc_error(
             -8, '{} already exists'.format(FILENAME),  node.dumptxoutset, FILENAME)