comprehensive series of testthat calls for both dbwritetable appends and also individual insert into calls

At the moment, there are several places where conversion happens. Also, different things for MonetDBLite vs. MonetDB.R. Starting points are here https://github.com/hannesmuehleisen/MonetDBLite/blob/Jul2017Lite-R/R/dbi.R#L770 and here https://github.com/hannesmuehleisen/MonetDBLite/blob/Jul2017Lite-R/src/embedded/converters.c.h

check DBItest too, we run those test cases now