Test and document limitations for distinct() or unique()

[ianmcook]

A useR!2017 attendee asked after the implyr talk about what Impala's limitations for using DISTINCT were. Older versions of Impala (before version 2.0) allowed only one DISTINCT clause per query. Newer versions have removed this limitation. But the current version of Impala does have the limitation that you cannot use DISTINCT in more than one aggregation function in the same query. Write tests to check these behaviors in implyr, and document the practical implications of these limitations for implyr users who are using the distinct() verb or the unique() function.