internal/protocol/json: Reduce unmarshal allocs

[bpot]

Check PkgPath on StructField which will be empty for exported fields
(http://golang.org/pkg/reflect/#StructField).

We have an application which heavily uses DynamoDB and we are seeing a lot of allocations in jsonutil/unmarshal. Based on a heap dump nearly 25% of the allocations (program wide) are coming from strings.ToLower in unmarshalStruct:

(pprof) peek ToLower
241578716 of 252105072 total (95.82%)
Dropped 547 nodes (cum <= 1260525)
----------------------------------------------------------+-------------
      flat  flat%   sum%        cum   cum%   calls calls% + context          
----------------------------------------------------------+-------------
                                          57705326   100% |   github.com/aws/aws-sdk-go/internal/protocol/json/jsonutil.unmarshalStruct
         0     0%     0%   57836399 22.94%                | strings.ToLower
                                          57836399   100% |   strings.Map
----------------------------------------------------------+-------------

[bpot]

This is the same technique that the json standard library uses (https://github.com/golang/go/blob/932c1e3dd32f636ab3f25b23d9dcef194a577bca/src/encoding/json/encode.go#L1025) to check if a field is export so it should be correct.

[jasdel]

Great find @bpot ! Thanks for submitting this PR. Allocations of the protocol marshallers is something we definitely want to improve. There are a series of benchmarks in internal/test/perf/protocol, but only for marshallers. None for unmarshalling have been added yet.

I noticed this issue is present in several of the other marshallers, so similar fixes might be able to be applied to them also. I created #377 to track improving the protocol marshallers in general.

[bpot]

Thanks!