feat(dup): implement ship_mutation stage and mutation_batch

[neverchanje]

See the previous PR: #304.

ship_mutation

ship_mutation is a part of the duplication procedure:

while true
do
  load_mutations | ship_mutations
done

It tries to ship mutations to remote via mutation_duplicator.

mutation_batch

mutation_batch is internally a prepare_list to ensure the sequential order
of loaded mutations.

[hycdong]

我对mutation_batch的功能还是有些疑问，mutation什么时候被加到热备的这个prepare_list里面的呢？是从private_log中load嘛？为什么还要对这个prepare_list进行prepare和commit呢？这个mutation_batch的用途我还是有些不明白

[neverchanje]

我对mutation_batch的功能还是有些疑问，mutation什么时候被加到热备的这个prepare_list里面的呢？是从private_log中load嘛？为什么还要对这个prepare_list进行prepare和commit呢？这个mutation_batch的用途我还是有些不明白

是的，在 plog load 的时候用。

prepare/commit 是因为热备份需要保证最终一致性，所以发送方需要保证按照日志的顺序发，
不能随便发，这里就是利用 prepare list 保证顺序性。

mutation_batch is internally a prepare_list to ensure the sequential order of loaded mutations.

// A sorted array of committed mutations that are ready for duplication.