[KafkaIO] Share size and offset estimators among ReadFromKafkaDoFn instances per PTransform

[sjvanrossum]

This change adds shared caches among unique instances of ReadFromKafkaDoFn, identified by a static AtomicLong which is stored as an instance field and incremented during construction of the DoFn. The constructor is bypassed during deserialization at execution time, thus providing a shared identifier for all instances of ReadFromKafkaDoFn for a single transform.

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