[Eager mode] Add an Eager-optimized version of the IteratorGetNext op.

Since in eager mode the kernel execution happens on the calling thread
(which would be blocked anyway), we can invoke the iterator
synchronously, and do not need to perform a context switch to and from
a background thread.

Improves the latency benchmarks in
//third_party/tensorflow/contrib/eager/python:datasets_test by
approximately 7us to 13us per element.

PiperOrigin-RevId: 183137488