gctrpc/order manager: Add ModifyOrder endpoint (#724)

* gctcli: modifyorder stubs

* gctcli: add ModifyOrderRequest and ModifyOrderResponse in rpc.proto

* gctcli: regenerate rpc.pb.go after the addition of ModifyOrder structs

* gctrpc: add ModifyOrder() and regenerate dependent files

* gctcli: modifyorder command now uses newly generated ModifyOrder() RPC

* exchanges/order/orders.go: use time.Time.Equal() instead of ==

* gctrpc: update ModifyOrderRequest and ModifyResponse and regenerate gRPC

* gctcli/commands: rework modifyorder

* engine: implement RPCServer.ModifyOrder

* engine: commit an initial version OrderManager.Modify(), still does not update state of managed orders

* engine: OrderManager.Modify now updates the inner state of managed orders, but introduces race conditions, needs fixes

* engine/order_manager.go: comply with golangci-lint

* gctcli: fix getOrderCommand.ArgsUsage

* gctcli: fix getModifyOrderCommand args and ArgsUsage

* engine: OrderManager.Modify() now correctly updates price of modified order

* engine: RPCServer.ModifyOrder now uses checkParams() as advised

* exchanges: (1) IBotExchange.ModifyOrder now returns a Modify struct, (2) all exchanges are updated to comply with that change

* exchanges/order: Detail.UpdateOrderFromModify also updates the ID

* engine/order_manager: add store.modifyExisting() and use it in OrderManager.Modify to update (on success) the state of the modified order

* exchanges: Bitfinex.ModifyOrder() now returns the ID in case of an error

* engine: OrdetManager.Modify() now emits an order event

* exchanges/bithumb: proper order.payment_currency key

* engine/order_manager: populate more Modify fields as they are needed by (some) exchanges, add comments

* engine: test OrderManager.Modify()

* engine: test store.modifyExisting()

* engine: write a docstring for store.modifyExisting

* engine: OrderManager.Modify() now also sets Modify.Price and Modify.Amount in case of zero values

* engine: TestOrderManager_Modify() now verify the effects of price and/or amount set to 0

* engine: OrderManger.Modify() now uses the commsManager to let observers know of errors

* engine: TestOrderManager_Modify() uses t.Fatal()

* engine: TestOrderManager_Modify() and TestStore_modifyOrder() supply t.Error() with proper messages

* exchanges/order_manager_test: fix a golangci-lint complaint

* engine/order_manager: fix an error comparison bug and simplify

* gctcli/commands: check if either price or amount is set, otherwise we would waste an API call

gctrpc/rpc.swagger.json

@@ -2114,6 +2114,80 @@
         ]
       }
     },
+    "/v1/modifyorder": {
+      "get": {
+        "operationId": "GoCryptoTrader_ModifyOrder",
+        "responses": {
+          "200": {
+            "description": "A successful response.",
+            "schema": {
+              "$ref": "#/definitions/gctrpcModifyOrderResponse"
+            }
+          },
+          "default": {
+            "description": "An unexpected error response.",
+            "schema": {
+              "$ref": "#/definitions/rpcStatus"
+            }
+          }
+        },
+        "parameters": [
+          {
+            "name": "exchange",
+            "in": "query",
+            "required": false,
+            "type": "string"
+          },
+          {
+            "name": "orderId",
+            "in": "query",
+            "required": false,
+            "type": "string"
+          },
+          {
+            "name": "pair.delimiter",
+            "in": "query",
+            "required": false,
+            "type": "string"
+          },
+          {
+            "name": "pair.base",
+            "in": "query",
+            "required": false,
+            "type": "string"
+          },
+          {
+            "name": "pair.quote",
+            "in": "query",
+            "required": false,
+            "type": "string"
+          },
+          {
+            "name": "asset",
+            "in": "query",
+            "required": false,
+            "type": "string"
+          },
+          {
+            "name": "amount",
+            "in": "query",
+            "required": false,
+            "type": "number",
+            "format": "double"
+          },
+          {
+            "name": "price",
+            "in": "query",
+            "required": false,
+            "type": "number",
+            "format": "double"
+          }
+        ],
+        "tags": [
+          "GoCryptoTrader"
+        ]
+      }
+    },
     "/v1/removeevent": {
       "post": {
         "operationId": "GoCryptoTrader_RemoveEvent",
@@ -4033,6 +4107,14 @@
         }
       }
     },
+    "gctrpcModifyOrderResponse": {
+      "type": "object",
+      "properties": {
+        "modifiedOrderId": {
+          "type": "string"
+        }
+      }
+    },
     "gctrpcOfflineCoinSummary": {
       "type": "object",
       "properties": {