`tf.train.experimental.ShardingCallback`s.

* `tf.train.CheckpointOptions`
    * Added `experimental_skip_slot_variables` (a boolean option) to skip
    restoring of optimizer slot variables in a checkpoint.

*   `tf.saved_model.SaveOptions`

    * `SaveOptions` now takes a new argument called
      `experimental_debug_stripper`. When enabled, this strips the debug nodes

  - build: Use go-runner:buster-v2.0.1 (built using go1.15.1)
  - bazel: Replace --features with Starlark build settings flag
  - hack/lib/util.sh: some bash cleanups
    
    - switched one spot to use kube::logging
    - make kube::util::find-binary return an error when it doesn't find
      anything so that hack scripts fail fast instead of with '' binary not
      found errors.

1F998..1F9A2  ; valid                  ;      ; NV8    # 11.0 KANGAROO..SWAN
1F9A3..1F9A4  ; valid                  ;      ; NV8    # 13.0 MAMMOTH..DODO
1F9A5..1F9AA  ; valid                  ;      ; NV8    # 12.0 SLOTH..OYSTER
1F9AB..1F9AD  ; valid                  ;      ; NV8    # 13.0 BEAVER..SEAL
1F9AE..1F9AF  ; valid                  ;      ; NV8    # 12.0 GUIDE DOG..PROBING CANE