store: refactor access hash and session tables

* Move sessions to user_login metadata, as that data rarely changes after login.
* Merge user and channel access hashes. Those IDs don't conflict.
* Split usernames into a new table to allow better `ON CONFLICT` updates
  (when a username moves to another entity, we want the old row to be replaced).
  Usernames also don't need to be scoped to a login.

pkg/connector/store/upgrades/00-latest.sql

@@ -1,12 +1,7 @@
--- v0 -> v3: Latest revision
-
-CREATE TABLE telegram_session (
-    user_id      BIGINT PRIMARY KEY,
-    session_data BYTEA NOT NULL
-);
+-- v0 -> v1: Latest revision
 
 CREATE TABLE telegram_user_state (
-    user_id BIGINT PRIMARY KEY,
+    user_id BIGINT NOT NULL PRIMARY KEY,
     pts     BIGINT NOT NULL,
     qts     BIGINT NOT NULL,
     date    BIGINT NOT NULL,
@@ -14,39 +9,35 @@ CREATE TABLE telegram_user_state (
 );
 
 CREATE TABLE telegram_channel_state (
-    user_id    BIGINT,
-    channel_id BIGINT,
+    user_id    BIGINT NOT NULL,
+    channel_id BIGINT NOT NULL,
     pts        BIGINT NOT NULL,
 
     PRIMARY KEY (user_id, channel_id)
 );
 
-CREATE INDEX idx_telegram_channel_state_user_id ON telegram_channel_state (user_id);
+CREATE INDEX telegram_channel_state_user_id_idx ON telegram_channel_state (user_id);
 
-CREATE TABLE telegram_channel_access_hashes (
-    user_id     BIGINT,
-    channel_id  BIGINT,
+CREATE TABLE telegram_access_hash (
+    user_id     BIGINT NOT NULL,
+    entity_id   BIGINT NOT NULL,
     access_hash BIGINT NOT NULL,
 
-    PRIMARY KEY (user_id, channel_id)
+    PRIMARY KEY (user_id, entity_id)
 );
 
-CREATE TABLE telegram_user_metadata (
-    receiver_id BIGINT,
-    user_id     BIGINT,
+CREATE TABLE telegram_username (
+    username  TEXT   NOT NULL,
+    entity_id BIGINT NOT NULL,
 
-    access_hash BIGINT NOT NULL,
-    username    TEXT,
-
-    PRIMARY KEY (receiver_id, user_id)
+    PRIMARY KEY (username)
 );
 
+CREATE INDEX telegram_username_entity_idx ON telegram_username (entity_id);
+
 CREATE TABLE telegram_file (
     id        TEXT PRIMARY KEY,
     mxc       TEXT NOT NULL,
     mime_type TEXT,
     size      BIGINT
 );
-
--- TODO this will be unnecessary once the queries switch to reading telegram_user_metadata
-CREATE INDEX idx_ghost_username ON ghost ((metadata->>'username'));