feat: add explicit fallthrough in the replication state transmission

[mzygQAQ]

I found that this project completely adopts c++17, so I'm not going to use the following tedious macro definitions：

#if HAS_CPP17_ATTRIBUTE(fallthrough)
#  define FALLTHROUGH [[fallthrough]]
#elif defined(__clang__)
#  define FALLTHROUGH [[clang::fallthrough]]
#elif GCC_VERSION >= 700 && \
    (!defined(__EDG_VERSION__) || __EDG_VERSION__ >= 520)
#  define FALLTHROUGH [[gnu::fallthrough]]
#else
#  define FALLTHROUGH
#endif

[PragmaTwice]

@guoxiangCN Thanks for your contribution!