[memory opt] Downsize decompression buffers when large for too long

[bimbashrestha]

#2066

[terrelln]

Please add a test, you should be able to use ZSTD_sizeof_DCtx() to test it.

[felixhandte]

Looks pretty good overall. Suggestions inline.

[Cyan4973]

Looks good to me. It's a clean patch, great work @bimbashrestha !
I believe the only remaining comment is from @terrelln , about a need to add a test to ensure the downsizing happens as expected (there is already such a test for CCtx, it could be extended to also cover DCtx).

And just FYI, since I did not participate on earlier discussion on this topic, I was not opposed on having separate constants for compression and decompression states, since it would allow differentiated policy if need be. But indeed, the names of constants should reflect their respective scope.

That being said, there's no urgency. We can start by using the same constants and therefore same policy for both DCtx and CCtx. We can always add new specific constants later if the need arises.

[terrelln]

LGTM once the test is added.