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, 12 insertions, 0 deletions

diff --git a/.sqlx/query-e718f4064cbb3d1b27049450121451dbcf01e8bec8a987b58c699b27b5d737af.json b/.sqlx/query-e718f4064cbb3d1b27049450121451dbcf01e8bec8a987b58c699b27b5d737af.json
new file mode 100644
index 0000000..0c21ec1
--- /dev/null
+++ b/.sqlx/query-e718f4064cbb3d1b27049450121451dbcf01e8bec8a987b58c699b27b5d737af.json
@@ -0,0 +1,12 @@
+{
+  "db_name": "SQLite",
+  "query": "\n                update message\n                set body = ''\n                where id = $1\n            ",
+  "describe": {
+    "columns": [],
+    "parameters": {
+      "Right": 1
+    },
+    "nullable": []
+  },
+  "hash": "e718f4064cbb3d1b27049450121451dbcf01e8bec8a987b58c699b27b5d737af"
+}