Revert "s3: reduced too greedy stack requirement for buffers causing segfault"

This reverts commit 75e56e3ec7438d9b6949f67c69f5f13506eb970d.
Revert "PLAT-355: S3 code now single threaded"

This reverts commit ef636f3c9f6fe2eb8f2e436eec974a804993020f.