Dbplyr joins fail with default suffixes

[Liudvikas-vinted]

To reproduce, join two tables that have column names in common but aren't the join key. Default suffixes (".x" and ".y") are attempted, but obviously column names in Impala (most SQLs I suspect) can't contain dots, so errors are thrown.
Default suffixes should be changed to something valid like "_x" and "_y" when we know we're joining Impala-backed tables.