Cancel the print process

[oterral]

Fix #2117

Test;

In the case a multiprint that send a cancel request to the print server.

• chsdi needs to be modify to handle the cancel request

[gjn]

FYI, there won't be a possibility to cancel a multiprint serverside because of multiple back-ends and cookie less state.

We could work around that using zadara back-end (create and check cancel file on shared volume), but I'm not sure it's worth the pain (given how rarelry the multiprint is used).

[procrastinatio]

@gjn I agree it is not very used, but with this PR, a single user may start, stop, change his mind and restart print jobs and destroy the server.

[gjn]

I'll create a PR tomorrow for back-end.

[gjn]

PR is back-end is ready. geoadmin/mf-chsdi3#1258

I've tested it with this branch and it works well. On deploy, also needs testing on int (because of multi-machine setup)

@procrastinatio @oterral Please test/review as well.

[oterral]

@procrastinatio can you review pls?

[procrastinatio]

Looks good for this part, I am testing the backend.

[oterral]

thx @procrastinatio