Add TF feature column ops for session-based recs

[karlhigley]

sequence_categorical_column_with_identity(...):
sequence_categorical_column_with_hash_bucket(...): A sequence of categorical terms where ids are set by hashing.
sequence_categorical_column_with_identity(...): Returns a feature column that represents sequences of integers.
sequence_categorical_column_with_vocabulary_file(...): A sequence of categorical terms where ids use a vocabulary file.
sequence_categorical_column_with_vocabulary_list(...): A sequence of categorical terms where ids use an in-memory list.
sequence_numeric_column(...): Returns a feature column that represents sequences of numeric data.
shared_embeddings(...): List of dense columns that convert from sparse, categorical input.

[marcromeyn]

Blocked by #763

[benfred]

shared_embeddings won't be possible - only works in graph mode