core: optimize group deletion (delays deletion of unused contacts) (#2560)

* core: optimize group deletion (wip)

* delay deletion of unused contacts

* clean up, fix test

* rename field

* remove from type, more checks, remove ctx

* remove space

* rename functions

* rename

---------

Co-authored-by: Evgeny Poberezkin <2769109+epoberezkin@users.noreply.github.com>

src/Simplex/Chat/Migrations/chat_schema.sql

@@ -63,6 +63,7 @@ CREATE TABLE contacts(
   contact_used INTEGER DEFAULT 0 CHECK(contact_used NOT NULL),
   user_preferences TEXT DEFAULT '{}' CHECK(user_preferences NOT NULL),
   chat_ts TEXT,
+  deleted INTEGER NOT NULL DEFAULT 0,
   FOREIGN KEY(user_id, local_display_name)
   REFERENCES display_names(user_id, local_display_name)
   ON DELETE CASCADE
@@ -652,3 +653,6 @@ CREATE INDEX idx_msg_deliveries_agent_ack_cmd_id ON msg_deliveries(
   connection_id,
   agent_ack_cmd_id
 );
+CREATE INDEX msg_delivery_events_msg_delivery_id ON msg_delivery_events(
+  msg_delivery_id
+);