Auto merge of #3080 - leto:jsindex, r=bitcartel

Return JoinSplit and JoinSplitOutput indexes in z_listreceivedbyaddress

This implements a feature request by @nathan-at-least in https://github.com/zcash/zcash/issues/2932 and I wanted to ask for feedback about the exact form of the data returned before updating tests and docs.

qa/rpc-tests/zkey_import_export.py

@@ -59,6 +59,12 @@ class ZkeyImportExportTest (BitcoinTestFramework):
 
             try:
                 assert_equal(amts, [tx["amount"] for tx in txs])
+                for tx in txs:
+                    # make sure JoinSplit keys exist and have valid values
+                    assert_equal( "jsindex" in tx, True)
+                    assert_equal( "jsoutindex" in tx, True)
+                    assert_greater_than( tx["jsindex"], -1 )
+                    assert_greater_than( tx["jsoutindex"], -1 )
             except AssertionError:
                 logging.error(
                     'Expected amounts: %r; txs: %r',