Add required thread synchronization to matrix kernel. (#280)

There is a potential data race in the `assemble_matrix_interleaved` kernel, where threads in a different warp can overwrite the `buffer_v` and `buffer_i` values before they are used to update the `d` and `rhs` vectors.

This race has been exercised in the asynchronous event delivery branch.

* Add `__syncthreads()` to assemble matrix interleaved kernel after `d` and `rhs` update.