lib: Fix foreign key bug introduced in d43235a

That commit used a boolean which caused an "ADD FOREIGN KEY" statement to be created
for each foreign key in a table if at least one of them needed to be added back after
dropping it to prevent conflicts. This commit fixes the logic so that only the
specific keys that were dropped get added back.

lib/TableDefinition.js

@@ -212,7 +212,7 @@ class TableDefinition {
     const operations = [];
     const oldForeignKeys = oldSchema.foreignKeys;
     const newForeignKeys = this._schema.foreignKeys;
-    var mustAddBackKey = false;
+    const keysToAddBack = new Set();
     var columnNames;
 
     for (columnNames in oldForeignKeys) {
@@ -222,7 +222,7 @@ class TableDefinition {
         if (!mustAvoidforeignKeyConflict(columnNamesArray, otherOperations)) {
           continue;
         }
-        mustAddBackKey = true;
+        keysToAddBack.add(columnNames);
       }
 
       const keyName = this._createKeyName(KEY_TYPES.FOREIGN, columnNamesArray);
@@ -234,7 +234,7 @@ class TableDefinition {
 
     for (columnNames in newForeignKeys) {
       const newForeignKeyData = newForeignKeys[columnNames];
-      if (!mustAddBackKey && isEqual(newForeignKeyData, oldForeignKeys[columnNames])) {
+      if (!keysToAddBack.has(columnNames) && isEqual(newForeignKeyData, oldForeignKeys[columnNames])) {
         continue;
       }
       const keySQL = this._generateForeignKeySQL(columnNames, newForeignKeyData);