*   Make the Bigtable client connection pool configurable & increase the
    default # of connections for performance.
*   Added derivative of `tf.random_gamma` with respect to the alpha parameter.
*   Added derivative of `tf.igamma(a, x)` and `tf.igammac(a, x)` with respect to
    a.
*   Modified Bessel functions of order zero and one.
*   Add FillTriangular Bijector to create triangular matrices.