Need to add support for batch operations

[HeyItsJs]

The grpc endpoint for batch operations is ready. Need to get that in client.

Here's how the batch API looks for javascript:

const batch = db.beginBatch();
batch.add(db.insert(col).docs(...docs));
batch.add(db.update(col).where(whereClause).set())

// Apply batch
batch.apply().then(res => ...).catch(ex => ...)

Same needs to be ported for Java

[HeyItsJs]

Needs to solve #3 before this