Fix memory leak of ArenaRpcPBMessageFactory #1516

Job	Run time
gcc-compile-with-make	2m 44s
gcc-compile-with-bazel	11m 15s
gcc-compile-with-boringssl	6m 35s
gcc-compile-with-cmake	3m 15s
gcc-compile-with-make-all-options	5m 5s
clang-compile-with-bazel	10m 2s
clang-compile-with-boringssl	8m 18s
clang-compile-with-cmake	2m 45s
clang-compile-with-make	3m 36s
clang-compile-with-make-all-options	5m 29s
gcc-compile-with-bazel-all-options	7m 21s
gcc-compile-with-cmake-all-options	6m 0s
clang-compile-with-bazel-all-options	7m 4s
clang-compile-with-cmake-all-options	7m 53s
clang-unittest	15m 22s
	1h 42m 44s

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