fix(datastore): increase deferred key iter limit (#2878)

A customer was hitting the upper bound for iteration through
deferred keys, and the logic underlying the 100 iteration limit
was flawed. Based on internal discussions and what's happening
in other clients, it's fine to increase this value.

datastore/datastore.go

@@ -421,15 +421,11 @@ func (c *Client) get(ctx context.Context, keys []*Key, dst interface{}, opts *pb
 	}
 	found := resp.Found
 	missing := resp.Missing
-	// Upper bound 100 iterations to prevent infinite loop.
-	// We choose 100 iterations somewhat logically:
-	// Max number of Entities you can request from Datastore is 1,000.
-	// Max size for a Datastore Entity is 1 MiB.
-	// Max request size is 10 MiB, so we assume max response size is also 10 MiB.
-	// 1,000 / 10 = 100.
+	// Upper bound 1000 iterations to prevent infinite loop. This matches the max
+	// number of Entities you can request from Datastore.
 	// Note that if ctx has a deadline, the deadline will probably
-	// be hit before we reach 100 iterations.
-	for i := 0; len(resp.Deferred) > 0 && i < 100; i++ {
+	// be hit before we reach 1000 iterations.
+	for i := 0; len(resp.Deferred) > 0 && i < 1000; i++ {
 		req.Keys = resp.Deferred
 		resp, err = c.client.Lookup(ctx, req)
 		if err != nil {