Replace non-customized asserts in concurrency (FSM) workloads

There are a few workloads that use the native assertion helpers instead of assertAlways/assertWhenOwnColl/assertWhenOwnDB:

count.js:            assert.gt(high, low);

remove_and_bulk_insert.js:            assert.writeOK(bulk.execute());

update_and_bulk_insert.js:            assert.writeOK(bulk.execute());

These assertions should be replaced to prevent test failures when workloads are composed together.