Clear stale sender certificates

ts/services/senderCertificate.ts

@@ -75,6 +75,24 @@ export class SenderCertificateService {
     return this.fetchCertificate(mode);
   }
 
+  // This is intended to be called when our credentials have been deleted, so any fetches
+  //   made until this function is complete would fail anyway.
+  async clear(): Promise<void> {
+    log.info(
+      'Sender certificate service: Clearing in-progress fetches and ' +
+        'deleting cached certificates'
+    );
+    await Promise.all(this.fetchPromises.values());
+
+    const { storage } = this;
+    assert(
+      storage,
+      'Sender certificate service method was called before it was initialized'
+    );
+    await storage.remove('senderCertificate');
+    await storage.remove('senderCertificateNoE164');
+  }
+
   private getStoredCertificate(
     mode: SenderCertificateMode
   ): undefined | SerializedCertificateType {