Support pthread mode for ExecutionQueue #536

Job	Run time
clang-compile-with-make	4m 59s
gcc-compile-with-cmake	4m 34s
gcc-compile-with-bazel-all-options	34m 20s
gcc-compile-with-bazel	32m 9s
gcc-compile-with-cmake-all-options	9m 44s
gcc-compile-with-make	4m 39s
gcc-compile-with-make-all-options	8m 43s
clang-compile-with-bazel	22m 20s
clang-compile-with-bazel-all-options	32m 42s
clang-compile-with-cmake	3m 11s
clang-compile-with-cmake-all-options	6m 10s
clang-compile-with-make-all-options	6m 41s
clang-unittest	17m 1s
	3h 7m 13s

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