BulkProcessor process every n+1 docs instead of n

When you set a BulkProcessor with a bulk actions size of 100, it executes the bulk after 101 documents.

```java
BulkProcessor.builder(client(), listener).setBulkActions(100).setConcurrentRequests(1).setName("foo").build();
```

Same for size. If you set the bulk size to 1024 bytes, it will actually execute the bulk after 1025 bytes.

This patch fix it.

Closes #4265.
(cherry picked from commit 43b5d91)