Adds redis cache for distributed environments (#2691)

* Makes schemaCache clearning promise-based

* Adds redis cache adapter for distributed systems

* Adds redis service to travis

* allow pg to fail

src/Adapters/Cache/RedisCacheAdapter.js

@@ -0,0 +1,69 @@
+import redis from 'redis';
+import logger from '../../logger';
+
+function debug() {
+  logger.debug.apply(logger, ['RedisCacheAdapter', ...arguments]);
+}
+
+export class RedisCacheAdapter {
+
+  constructor(ctx) {
+    this.client = redis.createClient(ctx);
+    this.p = Promise.resolve();
+  }
+
+  get(key) {
+    debug('get', key);
+    this.p = this.p.then(() => {
+      return new Promise((resolve, _) => {
+        this.client.get(key, function(err, res) {
+          debug('-> get', key, res);
+          if(!res) {
+            return resolve(null);
+          }
+          resolve(JSON.parse(res));
+        });
+      });
+    });
+    return this.p;
+  }
+
+  put(key, value, ttl) {
+    value = JSON.stringify(value);
+    debug('put', key, value, ttl);
+    this.p = this.p.then(() => {
+      return new Promise((resolve, _) => {
+        this.client.set(key, value, function(err, res) {
+          resolve();
+        });
+      });
+    });
+    return this.p;
+  }
+
+  del(key) {
+    debug('del', key);
+    this.p = this.p.then(() => {
+      return new Promise((resolve, _) => {
+        this.client.del(key, function(err, res) {
+          resolve();
+        });
+      });
+    });
+    return this.p;
+  }
+
+  clear() {
+    debug('clear');
+    this.p = this.p.then(() => {
+      return new Promise((resolve, _) => {
+        this.client.flushall(function(err, res) {
+          resolve();
+        });
+      });
+    });
+    return this.p;
+  }
+}
+
+export default RedisCacheAdapter;