set all transactions in block service to be serializable

accession-commons-monotonic-generator-jpa/src/main/java/uk/ac/ebi/ampt2d/commons/accession/persistence/jpa/monotonic/service/ContiguousIdBlockService.java

@@ -69,15 +69,15 @@ public ContiguousIdBlockService(ContiguousIdBlockRepository repository, Map<Stri
         this.categoryBlockInitializations = categoryBlockInitializations;
     }
 
-    @Transactional
+    @Transactional(isolation = Isolation.SERIALIZABLE)
     public void save(Iterable<ContiguousIdBlock> blocks) {
         // release block if full
         blocks.forEach(block -> {if (block.isFull()) {block.releaseReserved();}});
         repository.saveAll(blocks);
         entityManager.flush();
     }
 
-    @Transactional
+    @Transactional(isolation = Isolation.SERIALIZABLE)
     public void save(ContiguousIdBlock block) {
         // release block if full
         if (block.isFull()) {