Add flush after copy during downloads

[sswguo]

Trying to fix MMENG-4207 Inconsistent large downloads

Adding the flush ensures that all the data is sent to the client after the copy is complete. Without an explicit flush(), some data might remain in the buffer and not be sent until it's full.

[pkocandr]

I've noticed the cout is not closed anywhere in the code, so that's probably when flush happens normally without enforcing it. Although not sure how CountingOutputStream is implemented and if it is necessary to close it.

[sswguo]

Merging this for further tests, and to see if we need to explicitly close it too. thanks.