core: support down migrations to allow reverting to the previous version (#2072)

* core: support down migrations to allow reverting to the previous version

* update schema

* update simplexmq

* rename errors

* remove unused functions

* migration UI, test migration

* update migration UI

* return current migrations in CRVersionInfo

* update simplexmq

* test down migrations

* cleanup ios

* show migrations in log

src/Simplex/Chat/Migrations/chat_schema.sql

@@ -1,6 +1,7 @@
 CREATE TABLE migrations(
   name TEXT NOT NULL,
   ts TEXT NOT NULL,
+  down TEXT,
   PRIMARY KEY(name)
 );
 CREATE TABLE contact_profiles(