Fix mocking multiple http calls in the same function call

Fixes an issue where a function call would perform more than one http request and wait for each to complete before proceeding. The `RequestId` comes from the length of the `requests` collection in the `OffchainState` and if a request is completed before the next one starts it will be removed and the "next expected" will be off by one. This PR tries to fix that by using a request counter that tracks how many requests have been performed so that we can `remove()` items from the `expected_requests` at the right index.

I suspect that this is a sub-optimal soluton and perhaps requests and their mocks should live side by side in the same collection, e.g. in a tuple of `(PendingRequest, Option<ExpectedRequest>)`.