Update vendored dependencies

This includes github.com/kurin/blazer 0.2.0, which resolves #1291

vendor/cloud.google.com/go/spanner/mutation.go

@@ -126,7 +126,7 @@ type Mutation struct {
 	// op is the operation type of the mutation.
 	// See documentation for spanner.op for more details.
 	op op
-	// Table is the name of the taget table to be modified.
+	// Table is the name of the target table to be modified.
 	table string
 	// keySet is a set of primary keys that names the rows
 	// in a delete operation.

vendor/cloud.google.com/go/spanner/session.go

@@ -948,8 +948,12 @@ func (hc *healthChecker) worker(i int) {
 		ws := getNextForTx()
 		if ws != nil {
 			ctx, cancel := context.WithTimeout(context.Background(), time.Second)
-			defer cancel()
-			ws.prepareForWrite(contextWithOutgoingMetadata(ctx, hc.pool.md))
+			err := ws.prepareForWrite(contextWithOutgoingMetadata(ctx, hc.pool.md))
+			cancel()
+			if err != nil {
+				// TODO(dixiao): handle error properly
+				log.Errorf("prepareForWrite failed: %v", err)
+			}
 			hc.pool.recycle(ws)
 			hc.pool.mu.Lock()
 			hc.pool.prepareReqs--