[Redmine] Fix delay between requests in Redmine backend

There was no delay between requests in Redmine backend.
[Redmine] Retrieval issues from the latest analyzed

It uses the field 'updated_on' from the Redmine REST API.
Before r12477, Redmine API does not support dates in timestamp
format, just dates in "YY-MM-DD" format (i.e "2014-01-01").
Due to there is no way to know from which Redmine version we are
retrieving, we will use dates instead of timestamps for querying new issues.
[Redmine] Organize imports