Optimization of fastJsonNode

`fastJsonNode` is strongly refactored and optimized. Code is simpler, consumes less memory and executes faster.

Changes:
 - slices used instead of maps
 - unified `attrs` and `children` containers into one (`attrs`), removed fastJsonAttr type
 - (min-)heap used instead of sorting
 - much simplified processing (because of single container in `fastJsonNode`)

Performance comparison - artificial test (committed to `query/outputnode_test.go`), times measured without profiler. Memory measured (for entire test execution, including all setup) with default profiler configuration (because `-memprofilerate=1` was much too slow).

| Branch        | `master` | `improve/encode_meomry` | Improvement |
|------- -------|----------|-------------------------|-------------|
| Time:         | 172.283s | 0.336s                  | > 512 x     |
| Total memory" | 43.65GB  | 283.30MB                | > 157 x     |

In fact, memory improvement for encode function is much bigger - encode doesn't allocate memory. `fastJsonNode` creation is also much cheaper (in terms of memory).

This commit resolves #982, resolves #1138 and closes #1227.

query/common_test.go

@@ -158,11 +158,14 @@ func defaultContext() context.Context {
 }
 
 func processToFastJsonReq(t *testing.T, query string) (string, error) {
+	return processToFastJsonReqCtx(t, query, defaultContext())
+}
+
+func processToFastJsonReqCtx(t *testing.T, query string, ctx context.Context) (string, error) {
 	res, err := gql.Parse(gql.Request{Str: query, Http: true})
 	if err != nil {
 		return "", err
 	}
-	ctx := defaultContext()
 	queryRequest := QueryRequest{Latency: &Latency{}, GqlQuery: &res}
 	err = queryRequest.ProcessQuery(ctx)
 	if err != nil {