Increase database connection pool size to 20

The postgres.js default pool size is 10, which equals the concurrency
of ParallelMessageQueue.  Under federation load, all 10 workers can
simultaneously hold or wait for database connections, leaving none
available for HTTP handlers and KV store queries.  This causes the
server to stop responding to API requests while still answering
OPTIONS (CORS preflight) requests, which need no database access.

Increasing the pool size to 20 provides enough headroom for the 10
queue workers plus concurrent HTTP handlers and KV store operations.

#388

Author: dahlia

src/db.ts

@@ -13,6 +13,10 @@ const databaseUrl = process.env["DATABASE_URL"];
 if (databaseUrl == null) throw new Error("DATABASE_URL must be defined");
 
 export const postgres = createPostgres(databaseUrl, {
+  // The pool size needs to exceed the ParallelMessageQueue concurrency (10)
+  // to leave headroom for HTTP handlers and KV store queries.  The default
+  // of 10 can cause connection starvation under federation load.
+  max: 20,
   connect_timeout: 5,
   connection: { IntervalStyle: "iso_8601" },
 });