Consider computing checksum of uploaded parts

[brianhelba]

The UploadPart API allows an optional Content-MD5 header to be sent, containing:

The base64-encoded 128-bit MD5 digest of the part data.

For greater guarantees of data integrity, this digest could be computed locally by clients and included with uploads.

This could have a negative performance impact. However, it's possible that for small uploads, the actual extra time is negligible. For large uploads, it may be the case that the bottleneck is in upload bandwidth, so clever pipelining and parallelization will add only an initial constant (and thus amortizable) performance delay and will not actually scale with the upload size.

For the JS client, the most trusted choice of MD5 library seems to be crypto-js.