ColumnWriterImpl::write_batch_with_statistics incorrect distinct count in statistics

[tustvold]

Describe the bug

Calling write_batch_with_statistics twice with a non-zero distinct count will compute the sum of the distinct counts. In most cases this will be incorrect.

Similarly calling write_batch having called write_batch_with_statistics will not clear the distinct count.

To Reproduce

Inspect code

Expected behavior

We should only set distinct count when it is known

Additional context

#2015