Journal: don't attempt fetching entries if we are already up to date.

For a while now, etesync servers return the last entry uid as part of the
general journal querying. We can use this to check if we even need to
update the journals or not, making the overall sync much faster because
we no longer need to fetch journals that haven't changed.

app/src/main/java/com/etesync/syncadapter/App.kt

@@ -55,7 +55,7 @@ class App : Application() {
         get() = initDataStore()
 
     fun initDataStore(): MyEntityDataStore {
-        val source = MyDatabaseSource(this, Models.DEFAULT, 5)
+        val source = MyDatabaseSource(this, Models.DEFAULT, 6)
         val configuration = source.configuration
         return MyEntityDataStore(configuration)
     }

app/src/main/java/com/etesync/syncadapter/model/JournalModel.java

@@ -37,6 +37,8 @@ public class JournalModel {
 
         byte[] encryptedKey;
 
+        @Column(length = 64)
+        String remoteLastUid;
 
         @Deprecated
         long service;

app/src/main/java/com/etesync/syncadapter/syncadapter/SyncAdapterService.kt

@@ -247,6 +247,7 @@ abstract class SyncAdapterService : Service() {
                     journalEntity.encryptedKey = journal.key
                     journalEntity.isReadOnly = journal.readOnly
                     journalEntity.isDeleted = false
+                    journalEntity.remoteLastUid = journal.lastUid
                     data.upsert(journalEntity)
 
                     existing.remove(collection.uid)

app/src/main/java/com/etesync/syncadapter/syncadapter/SyncManager.kt

@@ -364,6 +364,12 @@ constructor(protected val context: Context, protected val account: Account, prot
                 }
             }
         } else {
+            if ((remoteCTag != null) && (journalEntity.remoteLastUid == remoteCTag)) {
+                Logger.log.info("Skipping fetch because local lastUid == remoteLastUid (${remoteCTag})")
+                remoteEntries = LinkedList()
+                return
+            }
+
             remoteEntries = journal!!.list(crypto, remoteCTag, MAX_FETCH)
         }