Improved threading performance on non-SQLite clients

refs https://github.com/TryGhost/Ghost/pull/12431

- child_process are more expensive to run comparing to worker_threads (ref. https://wanago.io/2019/05/20/node-js-typescript-14-performance-hooks/)
- limiting child process only to sqlite backed instances allows high load servers running with MySQL (most usecases) to continue using worker_threads

core/server/overrides.js

@@ -1,3 +1,5 @@
+const config = require('../shared/config');
+
 /**
  * If we enable bluebird debug logs we see a huge memory usage.
  * You can reproduce by removing this line and import a big database export into Ghost.
@@ -9,7 +11,10 @@ process.env.BLUEBIRD_DEBUG = 0;
  * Force bthreads to use child_process backend until a worker_thread-compatible version of sqlite3 is published
  * https://github.com/mapbox/node-sqlite3/issues/1386
  */
-process.env.BTHREADS_BACKEND = 'child_process';
+const isSQLite = config.get('database:client') === 'sqlite3';
+if (isSQLite) {
+    process.env.BTHREADS_BACKEND = 'child_process';
+}
 
 const moment = require('moment-timezone');