Added <dc:issued> field to OPDS entries

[veloman-yunkan]

As agreed in #702 (comment)

As of this PR the <updated> field in OPDS entries is not affected in any way. The same data is simply duplicated in a <dc:issued> field. The <updated> field will be changed later, after 2 conditions are met:

1. All respective client code is switched to use the <dc:issued> field
2. library.xml format is enhanced to contain information about book metadata timestamps

[codecov]

Codecov Report

Merging #715 (f8da503) into master (5c38300) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head f8da503 differs from pull request most recent head ae2d7d2. Consider uploading reports for the commit ae2d7d2 to get more accurate results

@@            Coverage Diff             @@
##           master     #715      +/-   ##
==========================================
+ Coverage   57.97%   57.99%   +0.02%     
==========================================
  Files          54       54              
  Lines        3569     3571       +2     
  Branches     2007     2009       +2     
==========================================
+ Hits         2069     2071       +2     
  Misses       1499     1499              
  Partials        1        1              

Impacted Files	Coverage Δ
src/book.cpp	86.66% <100.00%> (+0.18%)	⬆️
src/opds_dumper.cpp	99.07% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5c38300...ae2d7d2. Read the comment docs.

[mgautierfr]

All respective client code is switched to use the dc:issued field

Most of the client use libkiwix to parse the OPDS stream. It would be nice to also update the OPDS parsing part in this pr also.
It seems that the only part to change is https://github.com/kiwix/libkiwix/blob/master/src/book.cpp#L164

[veloman-yunkan]

All respective client code is switched to use the dc:issued field

Most of the client use libkiwix to parse the OPDS stream. It would be nice to also update the OPDS parsing part in this pr also. It seems that the only part to change is https://github.com/kiwix/libkiwix/blob/master/src/book.cpp#L164

@mgautierfr Done

[mgautierfr]

Agree. Please rebase-fixup before we merge.

[mgautierfr]

Agree. Please rebase-fixup before we merge.

Done