TSFetchRestpGet(), TSFetchPageResptGet() and TSFetchUrl() have incorrect asserts

The asserts cause TS to crash when these API functions are used. The asserts are incorrect as argument 'txnp' is actually not a transaction, but a FetchSM pointer as apparent in the code that follows.

And TSFetchUrl allows continuations to make requests with no callbacks and for such code paths, the continuation check should not be done.