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/schemas.spec.js

@@ -116,6 +116,11 @@ var masterKeyHeaders = {
 };
 
 describe('schemas', () => {
+
+  beforeEach(() => {
+     config.database.schemaCache.clear();
+  });
+
   it('requires the master key to get all schemas', (done) => {
     request.get({
       url: 'http://localhost:8378/1/schemas',