Support (store) ETag if remote (http) resource provides it

[yarikoptic]

https://en.wikipedia.org/wiki/HTTP_ETag

Loris provides ETags:

$> curl -v --output blah -G -H "Authorization: Bearer $token" https://openpreventad.loris.ca/api/v0.0.3-dev/candidates/1076159/NAPFU36/images/preventad_1076159_NAPFU36_fieldmap-phasediff_003.mnc
...
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Server: nginx/1.10.3
< Date: Tue, 11 Feb 2020 20:34:25 GMT
< Content-Type: application/x.minc2
< Content-Length: 431914
< Connection: keep-alive
< Access-Control-Allow-Origin: *
< Access-Control-Allow-Headers: *
< Content-Security-Policy: default-src 'self' 'unsafe-inline'; frame-src 'self' 'unsafe-inline' 'unsafe-eval' www.google.com  www.gstatic.com; script-src 'self' 'unsafe-inline' 'unsafe-eval' www.google.com  www.gstatic.com; font-src 'self' data:; img-src 'self' data:;
* Added cookie PHPSESSID="21mgvllpk8vo3rd45882tekduj" for domain openpreventad.loris.ca, path /, expire 0
< Set-Cookie: PHPSESSID=21mgvllpk8vo3rd45882tekduj; path=/; HttpOnly
< Expires: Thu, 19 Nov 1981 08:52:00 GMT
< Cache-Control: private
< Pragma: no-cache
< ETag: cf0b3b23cca79021e93bc0f753aea471
< Content-Disposition: attachment; filename=preventad_1076159_NAPFU36_fieldmap-phasediff_003.mnc
< 
{ [15604 bytes data]
100  421k  100  421k    0     0  1262k      0 --:--:-- --:--:-- --:--:-- 1262k
* Connection #0 to host openpreventad.loris.ca left intact

and then redownload could be prevented:

$> curl -v --output blah -G -H 'If-None-Match: cf0b3b23cca79021e93bc0f753aea471' -H "Authorization: Bearer $token" https://openpreventad.loris.ca/api/v0.0.3-dev/candidates/1076159/NAPFU36/images/preventad_1076159_NAPFU36_fieldmap-phasediff_003.mnc
...

and again example with HEAD request using curl

$> curl  -X HEAD --head -H "Authorization: Bearer $token" https://openpreventad.loris.ca/api/v0.0.3-dev/candidates/1076159/NAPFU36/images/preventad_1076159_NAPFU36_fieldmap-phasediff_003.mnc 
HTTP/1.1 200 OK
Server: nginx/1.10.3
Date: Tue, 11 Feb 2020 20:40:48 GMT
Content-Type: application/x.minc2
Content-Length: 431914
Connection: keep-alive
Access-Control-Allow-Origin: *
Access-Control-Allow-Headers: *
Content-Security-Policy: default-src 'self' 'unsafe-inline'; frame-src 'self' 'unsafe-inline' 'unsafe-eval' www.google.com  www.gstatic.com; script-src 'self' 'unsafe-inline' 'unsafe-eval' www.google.com  www.gstatic.com; font-src 'self' data:; img-src 'self' data:;
Set-Cookie: PHPSESSID=mmq99pr0f5s6a5q1gpmougha3a; path=/; HttpOnly
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: private
Pragma: no-cache
ETag: cf0b3b23cca79021e93bc0f753aea471
Content-Disposition: attachment; filename=preventad_1076159_NAPFU36_fieldmap-phasediff_003.mnc