Resolve "the 1.6.15 and 1.6.16 versions never starts the calculation of blocks"

Closes #1256 (closed)

app/lib/dal/fileDALs/PowDAL.ts

+import {AbstractCFS} from "./AbstractCFS"
+
+export class PowDAL extends AbstractCFS {
+
+  private static POW_FILE = "pow.txt"
+
+  constructor(rootPath:string, qioFS:any) {
+    super(rootPath, qioFS)
+  }
+
+  init() {
+    return this.coreFS.remove(PowDAL.POW_FILE, false).catch(() => {})
+  }
+
+  async getCurrent() {
+    return await this.coreFS.read(PowDAL.POW_FILE);
+  }
+
+  async writeCurrent(current:string) {
+    await this.coreFS.write(PowDAL.POW_FILE, current, false);
+  }
+}