Feature/add random sampling

[HyunsungJo]

• By placing an integer-type parameter "sample" in a step query, you can randomly sample N edges from the corresponding step result.
• Sampling from different steps in a multi-step query is also supported.
• Please refer to the query example below:

curl -XPOST localhost:9000/graphs/getEdges -H 'Content-Type: Application/json' -d '
{
    "srcVertices": [{"serviceName": "s2graph", "columnName": "a_id", "id":"1"}],
    "steps": [
      {"step": [{"label": "sampling_test_label", "direction": "out", "offset": 0, "limit": 10}], "sample": 2},  //  <== This will randomly draw two edges from step 1 and continue to step 2 with only those two cases.
      {"step": [{"label": "sampling_test_label", "direction": "out", "offset": 0, "limit": 10}]}
    ]
}'

curl -XPOST localhost:9000/graphs/getEdges -H 'Content-Type: Application/json' -d '
{
    "srcVertices": [{"serviceName": "s2graph", "columnName": "a_id", "id":"1"}],
    "steps": [
      {"step": [{"label": "sampling_test_label", "direction": "out", "offset": 0, "limit": 10}]},
      {"step": [{"label": "sampling_test_label", "direction": "out", "offset": 0, "limit": 10}], "sample": 2}  //  <== This will randomly draw two edges from each result sets of step 2 and return them as a final result.
    ]
}'

• This resolves #94.

[HyunsungJo]

• By placing an integer-type parameter "sample" as a QUERY PARAMETER, you can randomly sample N edges from the corresponding query result. (changes made in response to comments by @SteamShon)
• Sampling from different steps in a multi-step query is also supported.
• Sampling from results of different query parameters is also supported. (changes made in response to comments by @SteamShon)
• Please refer to the query example below:

curl -XPOST localhost:9000/graphs/getEdges -H 'Content-Type: Application/json' -d '
{
    "srcVertices": [{"serviceName": "s2graph", "columnName": "a_id", "id":"1"}],
    "steps": [
      {"step": [{"label": "sampling_test_label", "direction": "out", "offset": 0, "limit": 10, "sample": 2}]},  //  <== This will randomly draw two edges from step 1 and continue to step 2 with only those two cases.
      {"step": [{"label": "sampling_test_label", "direction": "out", "offset": 0, "limit": 10}]}
    ]
}'

curl -XPOST localhost:9000/graphs/getEdges -H 'Content-Type: Application/json' -d '
{
    "srcVertices": [{"serviceName": "s2graph", "columnName": "a_id", "id":"1"}],
    "steps": [
      {"step": [{"label": "sampling_test_label", "direction": "out", "offset": 0, "limit": 10}]},
      {"step": [{"label": "sampling_test_label", "direction": "out", "offset": 0, "limit": 10, "sample": 2}]}  //  <== This will randomly draw two edges from each result sets of step 2 and return them as a final result.
    ]
}'

curl -XPOST localhost:9000/graphs/getEdges -H 'Content-Type: Application/json' -d '
{
    "srcVertices": [{"serviceName": "s2graph", "columnName": "a_id", "id":"1"}],
    "steps": [
      {"step": [
          {"label": "sampling_test_label1", "direction": "out", "offset": 0, "limit": 10, "sample": 2}, 
          {"label": "sampling_test_label2", "direction": "out", "offset": 0, "limit": 10}
      ]}, //  <== Sampling only occurs for "sampling_test_label1"!
      {"step": [{"label": "sampling_test_label", "direction": "out", "offset": 0, "limit": 10}]}
    ]
}'

• This resolves #94.

[HyunsungJo]

continuing on #190..