Reduce peak memory usage during Lift Export

[johnthagen]

Currently when the backend exports a Lift file, it:

• Generates a zip file temporarily on disk (can be many 100s of MB)
• It reads this into memory
• It base64 decodes the contents in memory (which expands it ~30%)
• It sends this to the client in an HTTP response

This leads to peak memory usage of about (export_size x 2.3). It has been observed that for a 600MB export, system memory utilization increased to 2.2GB.

One option is to use NGINX sendfile to allow NGINX to efficiently stream the file back to the client and not load it into the backend memory:

• https://nginx.org/en/docs/http/ngx_http_core_module.html#sendfile

[johnthagen]

Related to #688 / #694