APIv2(DeliverMax): add alias for Amount in Payment transactions (#4733)

Using the "Amount" field in Payment transactions can cause incorrect
interpretation. There continue to be problems from the use of this
field. "Amount" is rarely the correct field to use; instead,
"delivered_amount" (or "DeliveredAmount") should be used.

Rename the "Amount" field to "DeliverMax", a less misleading name. With
api_version: 2, remove the "Amount" field from Payment transactions.

- Input: "DeliverMax" in `tx_json` is an alias for "Amount"
  - sign
  - submit (in sign-and-submit mode)
  - submit_multisigned
  - sign_for
- Output: Add "DeliverMax" where transactions are provided by the API
  - ledger
  - tx
  - tx_history
  - account_tx
  - transaction_entry
  - subscribe (transactions stream)
- Output: Remove "Amount" from API version 2

Fix #3484

Fix #3902

src/test/rpc/AccountTx_test.cpp

@@ -119,14 +119,23 @@ class AccountTx_test : public beast::unit_test::suite
         // Ledger 3 has the two txs associated with funding the account
         // All other ledgers have no txs
 
-        auto hasTxs = [](Json::Value const& j) {
+        auto hasTxs = [apiVersion](Json::Value const& j) {
             return j.isMember(jss::result) &&
                 (j[jss::result][jss::status] == "success") &&
                 (j[jss::result][jss::transactions].size() == 2) &&
                 (j[jss::result][jss::transactions][0u][jss::tx]
                   [jss::TransactionType] == jss::AccountSet) &&
                 (j[jss::result][jss::transactions][1u][jss::tx]
-                  [jss::TransactionType] == jss::Payment);
+                  [jss::TransactionType] == jss::Payment) &&
+                (j[jss::result][jss::transactions][1u][jss::tx]
+                  [jss::DeliverMax] == "10000000010") &&
+                ((apiVersion > 1 &&
+                  !j[jss::result][jss::transactions][1u][jss::tx].isMember(
+                      jss::Amount)) ||
+                 (apiVersion <= 1 &&
+                  j[jss::result][jss::transactions][1u][jss::tx][jss::Amount] ==
+                      j[jss::result][jss::transactions][1u][jss::tx]
+                       [jss::DeliverMax]));
         };
 
         auto noTxs = [](Json::Value const& j) {