download on demand (#2631)

* draft a download-api

* basic implementation

* allow partial downloads for protected chats

* use a separate column for download_state

* force a minimal timeout for delete_server_after in combination with partial messages

* add a warning if a possible download may expire by delete_server_after

* test load_imap_deletion_msgid()

* add a test for a partial download

* improve documentation and visibility

* let get_download_limit() return Result<Option>

* rusty getters

* apply MIN_DELETE_SERVER_AFTER to shown availability time

* move stub-creation to download.rs, use stock-strings, nicer logging

* make clippy happy (cargo clippy --tests)

* refine tests and comments

* fix typo

* remove superfluous closure in ffi

* respect partial_download for immediately scheduled DeleteMsgOnImap jobs

src/sql/migrations.rs

@@ -477,6 +477,16 @@ paramsv![]
         sql.execute_migration("UPDATE chats SET archived=1 WHERE blocked=2;", 78)
             .await?;
     }
+    if dbversion < 79 {
+        info!(context, "[migration] v79");
+        sql.execute_migration(
+            r#"
+        ALTER TABLE msgs ADD COLUMN download_state INTEGER DEFAULT 0;
+        "#,
+            79,
+        )
+        .await?;
+    }
 
     Ok((
         recalc_fingerprints,