SNOW-1640968 chunk downloader fix

[sfc-gh-dstempniak]

Description

Clearing memory after chunk download.

Checklist

• [x] Code compiles correctly
• [x] Code is formatted according to Coding Conventions
• [ ] Created tests which fail without the change (if possible)
• [x] All tests passing (dotnet test)
• [ ] Extended the README / documentation, if necessary
• [x] Provide JIRA issue id (if possible) or GitHub issue id in PR name

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.68508% with 6 lines in your changes missing coverage. Please review.

Project coverage is 85.86%. Comparing base (ef6db7e) to head (0f8ba07). Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
Snowflake.Data/Core/SFBlockingChunkDownloaderV3.cs	95.83%	3 Missing and 3 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1022 +/- ## ========================================== - Coverage 85.87% 85.86% -0.01% ========================================== Files 118 118 Lines 11455 11486 +31 Branches 1146 1148 +2 ========================================== + Hits 9837 9863 +26 - Misses 1333 1340 +7 + Partials 285 283 -2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.