Remove SolrCell support
Simplify document creation
In case of conflict, try later

If there is a conflict, trying later will solve the conflict by
either ignoring the task (old document) or executing it successfuly
(new document).
Remove useless date format

Date is automatically formated when placed in a document
Improve concurrent update with NRT

Use Near RealTime Search to do Optimistic concurrency, adding
the "_version_" field to the document ensure that the document hasn't
been modified between the "OudatedCheck" and the indexation of the
new version
Fix non-printable char issue with solr
Merge branch 'master' into search-1.5.x