Make sure (most) queries avoid table scans.

I've exempted inserts (they never scan in the first place), queries on `event_sequence` (at most one row), and the coalesce()s used for event replay (for now; these are obviously a performance risk area and need addressing).

Method:

```
find .sqlx -name 'query-*.json' -exec jq -r '"explain query plan " + .query + ";"' {} + > explain.sql
```

Then go query by query through the resulting file.

1 files changed, 0 insertions, 56 deletions

diff --git a/.sqlx/query-10a8d7ebb8a228c297f38e8ec70ecce6408cb241c9ac5f76bb484b214e720cb9.json b/.sqlx/query-10a8d7ebb8a228c297f38e8ec70ecce6408cb241c9ac5f76bb484b214e720cb9.json
deleted file mode 100644
index f8dd228..0000000
--- a/.sqlx/query-10a8d7ebb8a228c297f38e8ec70ecce6408cb241c9ac5f76bb484b214e720cb9.json
+++ /dev/null
@@ -1,56 +0,0 @@
-{
-  "db_name": "SQLite",
-  "query": "\n                select\n                    channel.id as \"id: Id\",\n                    name.display_name as \"display_name: String\",\n                    name.canonical_name as \"canonical_name: String\",\n                    channel.created_at as \"created_at: DateTime\",\n                    channel.created_sequence as \"created_sequence: Sequence\",\n                    deleted.deleted_at as \"deleted_at?: DateTime\",\n                    deleted.deleted_sequence as \"deleted_sequence?: Sequence\"\n                from channel\n                left join channel_name as name\n                    using (id)\n                left join channel_deleted as deleted\n                    using (id)\n                left join message\n                    on channel.id = message.channel\n                where channel.created_at < $1\n                    and message.id is null\n                    and deleted.id is null\n            ",
-  "describe": {
-    "columns": [
-      {
-        "name": "id: Id",
-        "ordinal": 0,
-        "type_info": "Text"
-      },
-      {
-        "name": "display_name: String",
-        "ordinal": 1,
-        "type_info": "Null"
-      },
-      {
-        "name": "canonical_name: String",
-        "ordinal": 2,
-        "type_info": "Null"
-      },
-      {
-        "name": "created_at: DateTime",
-        "ordinal": 3,
-        "type_info": "Text"
-      },
-      {
-        "name": "created_sequence: Sequence",
-        "ordinal": 4,
-        "type_info": "Integer"
-      },
-      {
-        "name": "deleted_at?: DateTime",
-        "ordinal": 5,
-        "type_info": "Text"
-      },
-      {
-        "name": "deleted_sequence?: Sequence",
-        "ordinal": 6,
-        "type_info": "Integer"
-      }
-    ],
-    "parameters": {
-      "Right": 1
-    },
-    "nullable": [
-      false,
-      true,
-      true,
-      false,
-      false,
-      true,
-      true
-    ]
-  },
-  "hash": "10a8d7ebb8a228c297f38e8ec70ecce6408cb241c9ac5f76bb484b214e720cb9"
-}