Move removed variants into a trash bin

[j-coll]

When removing the last study from a variant, the variant has to be removed, as it does not belong to any study any more. In order to keep track of the removed variants, and keep updated the SearchIndex (there may be other use cases in the future), those removed variants should be stored in some kind of "trash"

Storage MongoDB

When removing empty variants, the identifier should be moved to a new collection trash. The VariantMongoDBAdaptor should gain this methods:

+-------------------
| VariantMongoDBAdaptor
|-------------------
| + trashedVariants(timeStamp : long) : VariantDBIterator
| + cleanTrash(timeStamp : long) : long
+-------------------

Storage Hadoop

This does not apply yet to storage-hadoop, since the variants are never removed from the table, even if all the studies are removed.
Nothing to do.

Questions:

• What if we load a variant that is already in the trash bin?
  First remove trashed variants. Then load new variants. If a deleted variant is then loaded, and is in both collections, it will be first removed from solr and then loaded again, as the timestamp is greater than the current timestamp.