[Fix] Prevent ConcurrentModificationException in ScheduledContextRemo…

…ver.

The exception was likely caused by adding to the collection while it was being iterated during context removal.

src/main/java/cz/cvut/kbss/termit/persistence/dao/util/ScheduledContextRemover.java

@@ -54,7 +54,7 @@ public synchronized void scheduleForRemoval(@NonNull URI contextUri) {
      */
     @Transactional
     @Scheduled(fixedRate = 1, timeUnit = TimeUnit.MINUTES)
-    public void runContextRemoval() {
+    public synchronized void runContextRemoval() {
         LOG.trace("Running scheduled repository context removal.");
         contextsToRemove.forEach(g -> {
             LOG.trace("Dropping repository context {}.", Utils.uriToString(g));