Change the default value of the bthread_tag in class ServerOptions to… #1560

Job	Run time
clang-compile-with-bazel	15m 55s
clang-compile-with-bazel-all-options	16m 43s
clang-compile-with-cmake-all-options	4m 51s
clang-compile-with-cmake	2m 46s
clang-compile-with-boringssl	17m 8s
clang-compile-with-make	2m 52s
clang-compile-with-make-all-options	4m 50s
clang-unittest	15m 40s
gcc-compile-with-bazel	17m 16s
gcc-compile-with-bazel-all-options	17m 54s
gcc-compile-with-boringssl	17m 43s
gcc-compile-with-cmake	3m 42s
gcc-compile-with-cmake-all-options	5m 15s
gcc-compile-with-make	2m 49s
gcc-compile-with-make-all-options	4m 58s
	2h 30m 22s

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