Replace by fee storage

fixes #1222

backend/src/api/rbf-cache.ts

@@ -0,0 +1,34 @@
+export interface CachedRbf {
+  txid: string;
+  expires: Date;
+}
+
+class RbfCache {
+  private cache: { [txid: string]: CachedRbf; } = {};
+
+  constructor() {
+    setInterval(this.cleanup.bind(this), 1000 * 60 * 60);
+  }
+
+  public add(replacedTxId: string, newTxId: string): void {
+    this.cache[replacedTxId] = {
+      expires: new Date(Date.now() + 1000 * 604800), // 1 week
+      txid: newTxId,
+    };
+  }
+
+  public get(txId: string): CachedRbf | undefined {
+    return this.cache[txId];
+  }
+
+  private cleanup(): void {
+    const currentDate = new Date();
+    for (const c in this.cache) {
+      if (this.cache[c].expires < currentDate) {
+        delete this.cache[c];
+      }
+    }
+  }
+}
+
+export default new RbfCache();