distinct() problems for SQL sources

[krlmlr]

1. Doesn't seem to respect columns for SQL sources:

   > memdb_frame(a=1, b=2) %>% distinct(a, .keep_all = TRUE) %>% sql_render
   <SQL> SELECT DISTINCT *
   FROM `iksbzxasdl`

   Expected: SELECT DISTINCT(a)

2. Error with .keep_all = FALSE for SQL sources seems unnecessary, could be implemented with a second SELECT:

   > memdb_frame(a=1, b=2) %>% distinct(a) %>% sql_render
   <SQL> SELECT a FROM (SELECT DISTINCT(a)
   FROM `iksbzxasdl`)