change secondary commit to async execute in background thread

[levy5307]

Feature Request

Is your feature request related to a problem? Please describe:
In previous implementation, secondary will commit in next prepare request, which will increase latency of prepare request. I think we should add a rpc runs in background to do commit work, to decrease the latency of write request.

For more details: https://levy5307.github.io/blog/2pc/#pacifica

[foreverneverer]

I agree, however, its implementation may be more simple but no background task: we just need put prepare after the ack_prepare_message, for example:

    if (err == ERR_OK && status() != partition_status::PS_ERROR) {
        _private_log->append(mu, LPC_WRITE_REPLICATION_LOG_COMMON, &_tracker, nullptr);
    }
+ prepre.....

[levy5307]

I agree, however, its implementation may be more simple but no background task: we just need put prepare after the ack_prepare_message, for example:

    if (err == ERR_OK && status() != partition_status::PS_ERROR) {
        _private_log->append(mu, LPC_WRITE_REPLICATION_LOG_COMMON, &_tracker, nullptr);
    }
+ prepre.....

Sounds reasonable :）