Feature Request: Support async identity generation

[dmalanij]

I've been using WebApiThrottle for a while, for throttling requests on a SSO service and doing so I found some limitations on the way the identiy is generated. It would be really good to have it done asynchronously in case you depend on some I/O (in particular in my case, I needed to read some data from the request body).
This is a bit related with #44 and #76. I've already implemented some changes, will be sending a PR

[dmalanij]

Done, check #97. Hope it helps and happy to provide any clarification.