reuse_topic: Add index to timestamp bindings table (MaterializeInc#9379)

We want `compact_timestamp_bindings` to use only indexed lookups.  We can accumulate a large number of uncompacted timestamps if the sink enters into an errored state or the frontier stops moving forward for some reason.  Because of this, we want the queries in `compact_timestamp_bindings` to be completed in near constant-time.  Therefor, we add a `(sid, timestamp)` index so that we can do these lookups much more quickly.

src/coord/src/catalog/storage.rs

@@ -125,6 +125,10 @@ const MIGRATIONS: &[&str] = &[
     // Introduced in v0.9.12.
     "INSERT INTO schemas (database_id, name) VALUES
         (NULL, 'information_schema');",
+    // Adds index to timestamp table to more efficiently compact timestamps.
+    //
+    // Introduced in v0.12.0.
+    "CREATE INDEX timestamps_sid_timestamp ON timestamps (sid, timestamp)",
     // Add new migrations here.
     //
     // Migrations should be preceded with a comment of the following form: