Use evalsha for scripts

1. When constructor is called - execute a load scripts to all servers
2. Once we need to execute a script check if it’s loaded - if so do - evalsha otherwise fallback to eval

redlock.js

@@ -132,6 +132,8 @@ function Redlock(clients, options) {
 	this.servers = clients;
 	if(this.servers.length === 0)
 		throw new Error('Redlock must be instantiated with at least one redis server.');
+
+  this._loadScripts();
 }
 
 // Inherit all the EventEmitter methods, like `on`, and `off`
@@ -142,6 +144,49 @@ util.inherits(Redlock, EventEmitter);
 // to destinguish between error types.
 Redlock.LockError = LockError;
 
+Redlock.prototype._loadScripts = function () {
+  this.serverScripts = {};
+
+  const loadScriptsPromises = this.servers.map(
+    (server) => Promise.all([
+      this._loadScript(server, this.lockScript),
+      this._loadScript(server, this.unlockScript),
+      this._loadScript(server, this.extendScript),
+    ])
+    .then(scripts => {
+      this.serverScripts[server] = {
+        lockScript: scripts[0],
+        unlockScript: scripts[1],
+        extendScript: scripts[2]
+      };
+    }));
+
+  return Promise.all(loadScriptsPromises);
+}
+
+Redlock.prototype._loadScript = function(server, script) {
+  return new Promise((resolve, reject) => {
+    server.script('load', script, (err, result) => {
+      if(err !== null) {
+        reject(err);
+      } else {
+        resolve(result);
+      }
+    })
+  });
+}
+
+Redlock.prototype._executeScript = function(server, name, args, callback) {
+  if(this.serverScripts[server]) {
+    const scriptSha = this.serverScripts[server][name];
+    return server.evalsha(scriptSha, args, callback);
+
+  } else {
+    args.unshift(this[name]);
+    return server.eval(args, callback);
+  }
+}
+
 
 // quit
 // ----
@@ -232,15 +277,11 @@ Redlock.prototype.unlock = function unlock(lock, callback) {
 
 		// release the lock on each server
 		self.servers.forEach(function(server){
-			return server.eval(
-				[
-					self.unlockScript,
+      return self._executeScript(server, 'unlockScript', [
 					resource.length,
 					...resource,
 					lock.value
-				],
-				loop
-			)
+      ], loop);
 		});
 
 		function loop(err, response) {
@@ -341,32 +382,24 @@ Redlock.prototype._lock = function _lock(resource, value, ttl, callback) {
 		if(value === null) {
 			value = self._random();
 			request = function(server, loop){
-				return server.eval(
-					[
-						self.lockScript,
+        return self._executeScript(server, 'lockScript', [
 						resource.length,
 						...resource,
 						value,
 						ttl
-					],
-					loop
-				);
+        ], loop);
 			};
 		}
 
 		// extend an existing lock
 		else {
 			request = function(server, loop){
-				return server.eval(
-					[
-						self.extendScript,
+        return self._executeScript(server, 'extendScript', [
 						resource.length,
 						...resource,
 						value,
 						ttl
-					],
-					loop
-				);
+        ], loop);
 			};
 		}