Fasten the document deletion

When a document deletion occurs, instead of deleting the document we mark it as deleted
in the new “soft deleted” bitmap. It is then removed from the search, and all the other
endpoints.

milli/src/error.rs

@@ -27,6 +27,8 @@ pub enum Error {
 
 #[derive(Error, Debug)]
 pub enum InternalError {
+    #[error("Tried to access a soft deleted documents.")]
+    AccessingSoftDeletedDocument { document_id: DocumentId },
     #[error("{}", HeedError::DatabaseClosing)]
     DatabaseClosing,
     #[error("Missing {} in the {db_name} database.", key.unwrap_or("key"))]