add localstorage support for key backups

src/crypto/store/localStorage-crypto-store.js

@@ -32,6 +32,7 @@ const KEY_END_TO_END_ACCOUNT = E2E_PREFIX + "account";
 const KEY_DEVICE_DATA = E2E_PREFIX + "device_data";
 const KEY_INBOUND_SESSION_PREFIX = E2E_PREFIX + "inboundgroupsessions/";
 const KEY_ROOMS_PREFIX = E2E_PREFIX + "rooms/";
+const KEY_SESSIONS_NEEDING_BACKUP = E2E_PREFIX + "sessionsneedingbackup";
 
 function keyEndToEndSessions(deviceKey) {
     return E2E_PREFIX + "sessions/" + deviceKey;
@@ -165,6 +166,48 @@ export default class LocalStorageCryptoStore extends MemoryCryptoStore {
         func(result);
     }
 
+    getSessionsNeedingBackup(limit) {
+        const sessions = [];
+
+        for (const session in getJsonItem(this.store, KEY_SESSIONS_NEEDING_BACKUP)) {
+            const senderKey = session.substr(0, 43);
+            const sessionId = session.substr(44);
+            getEndToEndInboundGroupSession(senderKey, sessionId, null, (sessionData) => {
+                sessions.push({
+                    senderKey: senderKey,
+                    sessionId: sessionId,
+                    sessionData: sessionData,
+                });
+            })
+            if (limit && session.length >= limit) {
+                break;
+            }
+        }
+        return Promise.resolve(sessions);
+    }
+
+    unmarkSessionsNeedingBackup(sessions) {
+        const sessionsNeedingBackup = getJsonItem(this.store, KEY_SESSIONS_NEEDING_BACKUP) || {};
+        for(const session of sessions) {
+            delete sessionsNeedingBackup[session.senderKey + '/' + session.sessionId];
+        }
+        setJsonItem(
+            this.store, KEY_SESSION_NEEDING_BACKUP, sessionsNeedinBackup,
+        );
+        return Promise.resolve();
+    }
+
+    markSessionsNeedingBackup(sessions) {
+        const sessionsNeedingBackup = getJsonItem(this.store, KEY_SESSIONS_NEEDING_BACKUP) || {};
+        for(const session of sessions) {
+            sessionsNeedingBackup[session.senderKey + '/' + session.sessionId] = true;
+        }
+        setJsonItem(
+            this.store, KEY_SESSION_NEEDING_BACKUP, sessionsNeedinBackup,
+        );
+        return Promise.resolve();
+    }
+
     /**
      * Delete all data from this store.
      *