distsqlrun: add missing close call to backfiller

[dt]

Release note: none.

[cockroach-teamcity]

This change is 

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[vivekmenezes]

Seems okay, but then why have this in the first place if it's not fixing something?

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @lucy-zhang and @vivekmenezes)

[dt]

I'm not sure I follow: it is fixing something -- the missing close() that the interface stipulates must be called if prepare is called.

In this case that a) prints the on-close summary to the log and b) avoid leaking memory in the c++ SST builder on early error returns.

[vivekmenezes]

Thanks. LGTM!

[dt]

bors r+

[craig]

Build failed

• GitHub CI (Cockroach)

[dt]

🥐
bors r+

[craig]

Build succeeded

• GitHub CI (Cockroach)