decorateRequest is not thread safe

Due to the way the code for `decorateRequest` is written, when multiple requests are executed concurrently the [`currentAwsArguments` cache variable](https://github.com/fastify/aws-lambda-fastify/blob/main/index.js#L22) will clash and overwrite wrong event and context data between the simultaneous requests.

Start in index.js around line 22, where the currentAwsArguments cache variable used by decorateRequest is defined. Reproduce concurrent requests and trace how event and context data are paired; done means simultaneous requests no longer overwrite one another’s data.

Written by the indexing model from the issue text.