Allow multiple AsyncTasks to run concurrently

Fixes #134

app/src/main/java/org/isoron/uhabits/models/CheckmarkList.java

@@ -179,7 +179,7 @@ public abstract class CheckmarkList
      * @param from timestamp for the beginning of the interval
      * @param to   timestamp for the end of the interval
      */
-    protected final void compute(long from, final long to)
+    protected final synchronized void compute(long from, final long to)
     {
         final long day = DateUtils.millisecondsInOneDay;
 

app/src/main/java/org/isoron/uhabits/models/ScoreList.java

@@ -154,7 +154,7 @@ public abstract class ScoreList implements Iterable<Score>
      * @param from timestamp of the beginning of the interval
      * @param to   timestamp of the end of the time interval
      */
-    protected void compute(long from, long to)
+    protected synchronized void compute(long from, long to)
     {
         final long day = DateUtils.millisecondsInOneDay;
         final double freq = habit.getFrequency().toDouble();

app/src/main/java/org/isoron/uhabits/models/StreakList.java

@@ -65,7 +65,7 @@ public abstract class StreakList
 
     public abstract void invalidateNewerThan(long timestamp);
 
-    public void rebuild()
+    public synchronized void rebuild()
     {
         long today = DateUtils.getStartOfToday();
 

app/src/main/java/org/isoron/uhabits/tasks/AndroidTaskRunner.java

@@ -27,6 +27,8 @@ import java.util.*;
 
 import dagger.*;
 
+import static android.os.AsyncTask.*;
+
 @Module
 public class AndroidTaskRunner implements TaskRunner
 {
@@ -60,7 +62,7 @@ public class AndroidTaskRunner implements TaskRunner
     public void execute(Task task)
     {
         task.onAttached(this);
-        new CustomAsyncTask(task).execute();
+        new CustomAsyncTask(task).executeOnExecutor(THREAD_POOL_EXECUTOR);
     }
 
     @Override