Add a file size limit to etag SHA calculations, fallback to "Last modified" when above this limit.

[samuelclarkemindera]

Description

• Define a file size limit
• Any file above this does not use the etag header
• It will instead fallback to Last-Modified

Why

When downloading a large file (in my case 3 gig) the page seems to stall for a few minutes before the download begins.
This is due to the time it takes the etag SHA hash to be calculated with larger file sizes.

As a user this felt like something had silently errored.

Alternatives

• Implementing a faster hashing algorithm (The current crypto/node solution seems fairly quick as is, and requires no external lib)
• A different method of making the etag
• Do nothing but remember to use the --no-etag flag (good for those in the know, but bad for random folk who just want a quick file serving solution)