Integrate MyriaL support for sample() function

[senderista]

Looks like the sample() operator never made it into #428. I thought that the expected plans generated by sample() were equivalent to https://github.com/uwescience/myria/blob/master/jsonQueries/radion_queries/SampleWR.json and https://github.com/uwescience/myria/blob/master/jsonQueries/radion_queries/SampleWoR.json, but those plans just seem to be samplescan() with presampling. I think we actually need a new MyriaX reservoir sampling operator to properly support this function: uwescience/myria#859.