Support pg_bulkload

I have citus 11 env.

I used pg_bulkload to load the data into citus distributed table from csv file. this tool has better handling for bad parse data and duplicate too.

loading messages said no error, about 5mio records finished
but count the table always 0.

Try 2nd load; got error primary key constraint violation.

pg_bulkload I used is buffered writer, which in code they are using heap_insert API. Is citus parse data to distribute before heap_insert?