Convert Blockchain and Cache to es6, mv Cache to own file

Signed-off-by: Sina Mahmoodi <itz.s1na@gmail.com>

src/cache.js

@@ -0,0 +1,29 @@
+const LRU = require('lru-cache')
+
+// Simple LRU Cache that allows for keys of type Buffer
+module.exports = class Cache {
+  constructor (opts) {
+    this._cache = new LRU(opts)
+  }
+
+  set (key, value) {
+    if (key instanceof Buffer) {
+      key = key.toString('hex')
+    }
+    this._cache.set(key, value)
+  }
+
+  get (key) {
+    if (key instanceof Buffer) {
+      key = key.toString('hex')
+    }
+    return this._cache.get(key)
+  }
+
+  del (key) {
+    if (key instanceof Buffer) {
+      key = key.toString('hex')
+    }
+    this._cache.del(key)
+  }
+}