Specify timestamp fields in dist_test_tasks as nullable.
Specify timestamp fields in dist_test_tasks as nullable.

While trying this out on my laptop, MySQL 5.7 was erroring at the
CREATE TABLE since there was no default specified for these columns.

I think this might also be why our timestamps are messed up on the
production dist-test.cloudera.org, since we see negative task
and job durations until things complete (and all timestamp fields are
set).

umbrant/dist_test,umbrant/dist_test,cloudera/dist_test,cloudera/dist_test,cloudera/dist_test,umbrant/dist_test,umbrant/dist_test,cloudera/dist_test