benchmark rake task

namespace :benchmarks do
  namespace :dynamodb do
    task :get_item do

      require 'benchmark'
      require 'aws-sdk'

      table_name = 'aws-sdk-core-benchmarks'

      params = {
        table_name: table_name,
        key: {
          "id" => { s: "1" },
        },
        attributes_to_get: ["id"],
        consistent_read: false,
        return_consumed_capacity: "NONE",
      }

      json_params = {
        'TableName' => table_name,
        'Key' => {
          "id" => { 'S' => '1' },
        },
        'AttributesToGet' => ["id"],
        'ConsistentRead' => false,
        'ReturnConsumedCapacity' => "NONE",
      }

      ddb_v1 = AWS.dynamodb
      ddb_struct = Aws.dynamodb
      ddb_json = Aws.dynamodb(raw_json: true)

      ddb_v1.get_item(params)
      ddb_struct.get_item(params)
      ddb_json.get_item(json_params)

      label = "%10s"

      n = 50
      Benchmark.bm do |x|
        x.report(label % "v1")        { n.times { ddb_v1.get_item(params) }}
        x.report(label % "v2 Struct") { n.times { ddb_struct.get_item(params) }}
        x.report(label % "v2 JSON")   { n.times { ddb_json.get_item(json_params) }}
      end

    end
  end
end

benchmark results

Sample results with a run of 100 calls each:

```
$ rake benchmarks:dynamodb:get_item

       user     system      total        real
        v1  0.180000   0.010000   0.190000 (  1.011285)
 v2 Struct  0.190000   0.000000   0.190000 (  0.988390)
   v2 JSON  0.190000   0.000000   0.190000 (  0.794811)
```

Sample results with a run of 50 calls each:

```
$ rake benchmarks:dynamodb:get_item

       user     system      total        real
        v1  0.110000   0.010000   0.120000 (  0.366370)
 v2 Struct  0.080000   0.000000   0.080000 (  0.351170)
   v2 JSON  0.100000   0.010000   0.110000 (  0.342579)
```