region: Implement QueueBatch

Allows passing a batch of RPCs to the region client, which will be
added to the in progress multi and then sent to HBASE.

A note here is that it's possible for a multi to grow beyond the size
of the configured size. This feels like the right way to go because
the user of the Batch API has specifically requested the rpcs be
batched together, so splitting the batch across multiple multi's
breaks that bit of atomicity.

region/client.go

@@ -196,14 +196,7 @@ type client struct {
 // QueueRPC will add an rpc call to the queue for processing by the writer goroutine
 func (c *client) QueueRPC(rpc hrpc.Call) {
 	if b, ok := rpc.(hrpc.Batchable); ok && c.rpcQueueSize > 1 && !b.SkipBatch() {
-		// queue up the rpc
-		select {
-		case <-rpc.Context().Done():
-			// rpc timed out before being processed
-		case <-c.done:
-			returnResult(rpc, nil, ErrClientClosed)
-		case c.rpcs <- []hrpc.Call{rpc}:
-		}
+		c.QueueBatch(rpc.Context(), []hrpc.Call{rpc})
 	} else {
 		select {
 		case <-c.done:
@@ -223,6 +216,21 @@ func (c *client) QueueRPC(rpc hrpc.Call) {
 	}
 }
 
+// QueueBatch will add rpcs to the queue for processing by the writer
+// goroutine
+func (c *client) QueueBatch(ctx context.Context, rpcs []hrpc.Call) {
+	select {
+	case <-ctx.Done():
+	case <-c.done:
+		// return error results
+		res := hrpc.RPCResult{Error: ErrClientClosed}
+		for _, c := range rpcs {
+			c.ResultChan() <- res
+		}
+	case c.rpcs <- rpcs:
+	}
+}
+
 // Close asks this region.Client to close its connection to the RegionServer.
 // All queued and outstanding RPCs, if any, will be failed as if a connection
 // error had happened.