Add shift node so values from different times can be joined

[nathanielc]

Example from tests:

var period  = 5s

var data  = stream
    .from()
        .measurement('cpu')
        .where(lambda: "host" == 'serverA')

var past = data
    .window()
        .period(period)
        .every(period)
        .align()
    .mapReduce(influxql.count('value'))
    //shift values from the past forward so they can be compared with current values.
    .shift(period)

var current = data
    .window()
        .period(period)
        .every(period)
        .align()
    .mapReduce(influxql.count('value'))

past.join(current)
    .as('past', 'current')
    .eval(lambda: "current.count" - "past.count")
        .keep()
        .as('diff')
    .httpOut('TestStream_Shift')