Flush write state when initializing read state
Flush write state when initializing read state

In next commit, we will adjust curcid of the snapshot being used when
scanning the columnar table.

However, for index scan, snapshot is provided not when beginning scan
but within fetch-tuple call.

For this reason, start flushing pending writes in init_columnar_read_state
since this seem to be a prerequisite step that needs to be done before
scanning a columnar table regardless of the scan method being used.

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