Add option to re-use schema cache between requests (#2979)

* Add option to reuse database controller between requests. Clear schema cache when deleting everything

* Add test

* Rename setting to persistSchemaCache to more accurately reflect effect

* Repurpose option to determine whether to randomize cache prefix. Restore Config.js controller creation. Add tests

* Fix bug with missing parameter passed to to SchemaCache

* Renaming and formatting

* Fix property name typo

* Rename option to avoid double negative and still be falsey by default. Style fix

src/cli/definitions/parse-server.js

@@ -195,6 +195,11 @@ export default {
     help: "The TTL for caching the schema for optimizing read/write operations. You should put a long TTL when your DB is in production. default to 0; disabled.",
     action: numberParser("schemaCacheTTL"),
   },
+  "enableSingleSchemaCache": {
+    env: "PARSE_SERVER_ENABLE_SINGLE_SCHEMA_CACHE",
+    help: "Use a single schema cache shared across requests. Reduces number of queries made to _SCHEMA. Defaults to false, i.e. unique schema cache per request.",
+    action: booleanParser
+  },
   "cluster": {
     help: "Run with cluster, optionally set the number of processes default to os.cpus().length",
     action: numberOrBoolParser("cluster")