Release 0.21.0 (2018-12-19)

Baseline: cb9b2afbba3f8d3a1db8bf68e65d06f1b36902f5

Cherry picks:

   + 12b96466ee0d6ab83f7d4cd24be110bb5021281d:
     Windows, test wrapper: rename the associated flag
   + 7fc967c4d6435de2bb4e34aac00ca2e499f55fca:
     Use a fixed thread pool in ByteStreamBuildEventArtifactUploader
   + 798b9a989aa793655d29504edb5fb85f3143db84:
     Add --build_event_upload_max_threads option
   + dbe05df23ccf4c919379e0294e0701fd3f66739c:
     Update the version of  skylib bundled in the distfile

Incompatible changes:

  - The --experimental_stl command line option is removed.
  - aquery defaults to human readable output format.

New features:

  - repository_ctx.download and repository_ctx.download_and_extract
    now return a struct.
  - Android Databinding v2 can be enabled with
    --experimental_android_databinding_v2.

Important changes:

  - The deprecated and unmaintained Docker rules in
    tools/build_defs/docker were removed. Please use
    https://github.com/bazelbuild/rules_docker instead.
  - The new --upload_query_output_using_bep query/cquery/aquery flag
    causes query outputs to be uploaded via BEP.
  - New incompatible flag --incompatible_strict_argument_ordering
  - --strict_android_deps and --strict_java_deps were renamed to
    --experimental_strict_java_deps
  - config_settings that select on "compiler" value instead of values
    = {"compiler" : "x"} should use flag_values =
    {"@bazel_tools//tools/cpp:compiler": "x"}.
  - The new --upload_query_output_using_bep query/cquery/aquery flag
    causes query outputs to be uploaded via BEP.
  - Turn on --incompatible_disable_sysroot_from_configuration
  - We revamped our Android with Bazel tutorial! Check it out
    [here](https://docs.bazel.build/versions/master/tutorial/android-a
    pp.html).
  - --incompatible_disallow_slash_operator is now on by default
  - Enable --experimental_check_desugar_deps by default.  This flag
    rules out several types of invalid Android builds at compile-time.
  - The --max_config_changes_to_show option lists the names of
    options which
    have changed and thus caused the analysis cache to be dropped.
  - The --experimental_strict_action_env option has been renamed to
    --incompatible_strict_action_env and is now on by default. This
    means Bazel will no longer use the client's PATH and
    LD_LIBRARY_PATH environmental variables in the default action
    environment. If the old behavior is desired, pass
    --action_env=PATH and --action_env=LD_LIBRARY_PATH.
    --noincompatible_strict_action_env will also temporarily restore
    the old behavior. However, as --action_env is a more general and
    explicit way to pass client environmental variables into actions,
    --noincompatible_strict_action_env will eventually be deprecated
    and removed. See #6648 for more details.
  - XCRUNWRAPPER_LABEL has been removed. If you used this value
    before, please use @bazel_tools//tools/objc:xcrunwrapper instead.
  - --incompatible_static_name_resolution is no unable by default
  - We will phase out --genrule_strategy in favor of
    --strategy=Genrule=<value> (for genrules) or
    --spawn_strategy=<value> (for all actions).
  - --incompatible_package_name_is_a_function is now enabled by
    default
  - Dynamic execution is now available with
    --experimental_spawn_strategy. Dynamic execution allows a build
    action to run locally and remotely simultaneously, and Bazel
    picks the fastest action. This provides the best of both worlds:
    faster clean builds than pure local builds, and faster
    incremental builds than pure remote builds.
  - --incompatible_package_name_is_a_function is now enabled by
    default
  - New incompatible flag --incompatible_merge_genfiles_directory
  - grpc log now logs updateActionResult
  - CppConfiguration doesn't do package loading anymore. That means:
    * it's no longer needed to have C++ toolchain available when
    building non-C++ projects
    * bazel will not analyze C++ toolchain when not needed -> speedup
    ~2s on bazel startup when C++ rules using hermetic toolchain are
    not loaded
  - --incompatible_package_name_is_a_fu...

This release contains contributions from many people at Google, as well as andy g scott ?, Attila Ol?h, Benjamin Peterson, Clint Harrison, Dave Lee, Ed Schouten, Greg Estren, Gregor Jasny, Jamie Snape, Jerry Marino, Loo Rong Jie, Or Shachar, Sevki Hasirci, William Chargin.