Use batch writes from Fabric 3.1

In the method `.flushWrites()` which is called at the end of the transaction use batch writes which are supported from Fabric `3.1`. They can be used for other versions, it just has no effect. As documented in the Fabric source code in `interfaces.go`:

```go
// StartWriteBatch enables a mode where all changes are not immediately forwarded to the peer,
// but accumulate in the cache. The cache is sent in large batches either at the end of transaction
// execution or after the FinishWriteBatch call.
// IMPORTANT: in this mode, the expected order of transaction execution and expected errors can be changed.
//
// If write batching is not supported by the peer, this method has no effect
// and writes to the ledger continue to be processed immediately.
StartWriteBatch()

// FinishWriteBatch sends accumulated changes in large batches to the peer
// if StartWriteBatch has been called before it.
//
// If write batching is not supported by the peer or no write batch has been
// started, this method has no effect and returns nil.
FinishWriteBatch() error
```