fix(bulk_load): fix bug that poping all committed mutations ineffective

[hycdong]

Pegasus#872 In bulk load process, all refactors of replica will download sst files and ingest them into rocksdb. After the ingestion, we will send an empty write and pop all committed mutations in prepare list to let learn process will always start from learn checkpoint.
However, according to the community usage, we found that pop all committed mutation not work, this pull request fixs it, including:

• Sending another empty write after generate checkpoint. The formal first empty write is used to let secondary commit ingestion request, the new secondary empty write is to do real pop action.
• Secondary replica will also execute pop all committed mutation action, which is not implemented in old version.