lock(key, ttl = 15000) {
		if (this.redlock == null) {
			return this._handleMissingRedlock();
		}

		key = this.prefix + key + "-lock";
		return this.redlock.lock(key, ttl).then(lock => {
			return () => lock.unlock();
		});
	}
Acquire a lock

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

@memberof RedisCacher