* 🚀 Feature: Add idempotency middleware (#2253)
* middleware: add idempotency middleware
* middleware/idempotency: use fiber.Storage instead of custom storage
* middleware/idempotency: only allocate data if really required
* middleware/idempotency: marshal response using msgp
* middleware/idempotency: add msgp tests
* middleware/idempotency: do not export response
* middleware/idempotency: disable msgp's -io option to disable generating unused methods
* middleware/idempotency: switch to time.Duration based app.Test
* middleware/idempotency: only create closure once
* middleware/idempotency: add benchmarks
* middleware/idempotency: optimize strings.ToLower when making comparison
The real "strings.ToLower" still needs to be used when storing the data.
* middleware/idempotency: safe-copy body
* middleware/idempotency: backport to v2
leonklingele