Adds schema caching capabilities (5s by default) (#2286)

* Adds schema caching capabilities (off by default)

* Use InMemoryCacheAdapter

* Uses proper adapter to generate a cache

* Fix bugs when running disabled cache

* nits

* nits

* Use options object instead of boolean

* Imrpove concurrency of loadSchema

* Adds testing with SCHEMA_CACHE_ON

* Use CacheController instead of generator

- Makes caching SchemaCache use a generated prefix
- Makes clearing the SchemaCache clear only the cached schema keys
- Enable cache by default (ttl 5s)

spec/PointerPermissions.spec.js

@@ -4,6 +4,11 @@ var Schema = require('../src/Controllers/SchemaController');
 var Config = require('../src/Config');
 
 describe('Pointer Permissions', () => {
+
+  beforeEach(() => {
+    new Config(Parse.applicationId).database.schemaCache.clear();
+  });
+
   it_exclude_dbs(['postgres'])('should work with find', (done) => {
     let config = new Config(Parse.applicationId);
     let user = new Parse.User();