Add support for async serializer so it can be used with worker threads

[ozcanovunc]

Hi @tj

In our use case, we use huge sessions. That's why we needed to use worker threads for parsing.
In order to achieve that, we needed async parse function to be sent into worker thread.

This change won't break for non-async parse functions.

Kindly review
Cheers

[wavded]

Sorry for the delay on this. This looks good. Could you update the documentation as well?

[ozcanovunc]

Hey no worries, done :)

[wavded]

Thx!

[wavded]

Published in https://github.com/tj/connect-redis/releases/tag/v7.1.0