all: add support for topics and refactor other things

pkg/connector/store/upgrades/06-topic-index.sql

@@ -0,0 +1,8 @@
+-- v6 (compatible with v2+): Add table for topics
+
+CREATE TABLE telegram_topic (
+    channel_id BIGINT NOT NULL,
+    topic_id   BIGINT NOT NULL,
+
+    PRIMARY KEY (channel_id, topic_id)
+);