Allow auto calculated limit to be inserted a specific place instead of just appended to the end

[ptomasroos]

What this PR does / why we need it:

Which issue(s) this PR fixes:
Fixes #232

Special notes for your reviewer:

Release note:

This allows the user to specify where the LIMIT statement should be inserted when using $__timeGroup(..., auto).

Use $__limitPosition to specify if the limit should be inserted to a specific placement rather than the default behaviour of appending to the end of the query.

[LiorRacer]

Hi @ptomasroos,
Looks good can you please add a test for it?

[ptomasroos]

There are no tests covering the LIMIT logic today, so can you give me a suggestion on how I would test this?

[LiorRacer]

I would call the query method like so:

describe("When performing do request with limit position", () => {
    const limitResult = 2;
    let results;
    const response = {
      kind: "bigquery#queryResponse",
      schema: {
        fields: [
          {
            name: "time",
            type: "TIMESTAMP",
            mode: "NULLABLE"
          },
          {
            name: "start_station_latitude",
            type: "FLOAT",
            mode: "NULLABLE"
          }
        ]
      },
      jobReference: {
        projectId: "aviv-playground",
        jobId: "job_fB4qCDAO-TKg1Orc-OrkdIRxCGN5",
        location: "US"
      },
      totalRows: "3",
      rows: [
        {
          f: [
            {
              v: "1.521578851E9"
            },
            {
              v: "37.7753058"
            }
          ]
        },
        {
          f: [
            {
              v: "1.521578916E9"
            },
            {
              v: "37.3322326"
            }
          ]
        },
        {
          f: [
            {
              v: "1.521578927E9"
            },
            {
              v: "37.781752"
            }
          ]
        }
      ],
      totalBytesProcessed: "23289520",
      jobComplete: true,
      cacheHit: false
    };
    beforeEach(() => {
      ctx.backendSrv.datasourceRequest = jest.fn(options => {
        return Promise.resolve({ data: response, status: 200 });
      });
    });

    it("should return expected amount of rows", async () => {    
      await ctx.ds.query({}) //=> call with your params
      expect(results.rows.length).toBe(limitResult);
    });
  });