tryLock(key, ttl) {
		if (this._lock.isLocked(key)) {
			return this.broker.Promise.reject(new Error("Locked."));
		}
		return this._lock.acquire(key, ttl).then(() => {
			return () => this._lock.release(key);
		});
	}
Try to acquire a lock

@param {string|Array<string>} key
@param {Number} ttl Optional Time-to-Live
@returns {Promise}

@memberof MemoryLRUCacher