Allow "COUNTER" batches (transactions)

The implementation of the counter batches here utilizes a separate function to the existing ```beginBatch()``` function.

Another implementation could be to extend the existing function to take an optional "type" parameter which would be limited to values of "COUNTER" and "UNLOGGED' (see https://cassandra.apache.org/doc/cql3/CQL.html#batchStmt).
The batch query line would then become...
```php
$this->batchQuery = "BEGIN $type BATCH\n";
```

I will be performing a lot of counter updates for specific events in an application and so Counter Batches are extremely useful to me... I believe this will provide additional value to other clients also.

Regards,
Adam