Investigate reducing memory usage of chunk uploading

[szokeasaurusrex]

Chunk uploading appears to have a very large memory usage when uploading large files, since we appear to be reading the entire file contents into memory. For instance, the (currently experimental) Proguard chunk uploading feature greatly increases memory usage versus when the experimental chunk uploading feature is turned off. While profiling an upload of a 160 MB Proguard file, I observed memory usage of >300 MB while chunk uploading, vs ~90 MB with chunk uploading disabled.

As this problem likely affects all chunk uploads, not just Proguard files, we should investigate if we can reduce memory usage, e.g. by not reading the entire files into memory.