synchronize pEpEngine init to avoid serialize session init-s

src/org/pEp/jniadapter/AbstractEngine.java

@@ -24,8 +24,10 @@ abstract class AbstractEngine implements AutoCloseable {
     }
 
     public AbstractEngine() throws pEpException {
+        synchronized (AbstractEngine.class) {
             init();
         }
+    }
 
     final public void close() {
         release();