Async functions do not return correct values

# Problem This library exposes many async functions that swallow any returned data or errors from redis if you try to use them as a promise instead of a callback function. These functions should return data and errors in the fashion of a promise as well. ### Functions in Question - get: [./index.ts#L99](https://github.com/tj/connect-redis/blob/master/index.ts#L99) - set: [./index.ts#L110](https://github.com/tj/connect-redis/blob/master/index.ts#L110) - touch: [./index.ts#L127](https://github.com/tj/connect-redis/blob/master/index.ts#L127) - destroy: [./index.ts#L138](https://github.com/tj/connect-redis/blob/master/index.ts#L138) - clear: [./index.ts#L148](https://github.com/tj/connect-redis/blob/master/index.ts#L148) - length: [./index.ts#L159](https://github.com/tj/connect-redis/blob/master/index.ts#L159) - ids: [./index.ts#L168](https://github.com/tj/connect-redis/blob/master/index.ts#L168) - all: [./index.ts#L181](https://github.com/tj/connect-redis/blob/master/index.ts#L181) ## Proposed Solution Just make the callback function optional. If it's present, have it behave as it does right now. If it's not, let it behave as a promise. That way both callbacks and promises work as expected without breaking existing behaviors. Just make a helper function to wrap cbs with: ```javascript function optionalCb(err: unknown, data: unknown, cb: Function) { if (cb) return cb(err, data) if (err) throw err return data } ``` And this is how you would use it E.g. the get function ```javascript async get(sid: string, cb?: Function) { let key = this.prefix + sid try { let data = await this.client.get(key) if (!data) return optionalCb(null, null, cb) return optionalCb(null, await this.serializer.parse(data), cb) } catch (err) { return optionalCb(err, null, cb) } } ``` The helper function is literally a drop in replacement for where you normally call `cb()` so it should be a pretty minor change.