Clean-up sync artifacts in Smarti-MongoDB

[ruKurz]

The DELETE operation does mark the conversation as deleted AND deletes all messages. The conversation and its metadata are preserved as those information are required for cloud-sync of the embedded Solr Servers managed by the different Smarti instances.

For clean up one could run a Mongo task that physically deletes all conversations that where marked as deleted for longer as a given time period (e.g. 1 day).

NOTES:

1. Even when using an external Solr those Information are critical to sync the Index after a down period (as one needs to know what conversations need to be deleted from the index).
2. When a clean up process is in place Solr Indexes that are out of sync for longer as the configured time period of the clean up process need to be deleted and fully rebuild.

#257

[westei]

When we can use Mongo TTL indexes we will implement this within #257

[westei]

Added this to the Solr Update project, as Cloudsync will go away when using an external Solr Server and therefore this is also related to this update.