Improved support for UNION

[systay]

Planner improvements that allow UNION to be solved in a single route instead of being sent as separate queries and the concatenated at the vtgate level.

Example query:

select id from user 
union all 
select id from music

Before this change, this would have been solved by sending two scatter queries to all shards - one querying for user and one for music.

{
  "QueryType": "SELECT",
  "Original": "select id from user union all select id from music",
  "Instructions": {
    "OperatorType": "Concatenate",
    "Inputs": [
      {
        "OperatorType": "Route",
        "Variant": "SelectScatter",
        "Keyspace": {
          "Name": "user",
          "Sharded": true
        },
        "FieldQuery": "select id from user where 1 != 1",
        "Query": "select id from user",
        "Table": "user"
      },
      {
        "OperatorType": "Route",
        "Variant": "SelectScatter",
        "Keyspace": {
          "Name": "user",
          "Sharded": true
        },
        "FieldQuery": "select id from music where 1 != 1",
        "Query": "select id from music",
        "Table": "music"
      }
    ]
  }
}

With this change, the planner can see that both tables live on the same shards, so this query can be executed with fewer network calls.

{
  "QueryType": "SELECT",
  "Original": "select id from user union all select id from music",
  "Instructions": {
    "OperatorType": "Route",
    "Variant": "SelectScatter",
    "Keyspace": {
      "Name": "user",
      "Sharded": true
    },
    "FieldQuery": "select id from user where 1 != 1 union all select id from music where 1 != 1",
    "Query": "select id from user union all select id from music",
    "Table": "user"
  }
}