Need an API for distributedlog AppendOnlyWriter that only care about the data is already flushed but not committed.

[YutSean]

FEATURE REQUEST

1. We are using the distributedlog as a WAL storage. Currently, the AppendOnlyWriter only support a force method to make sure the data is already persistently stored in BK.

2. The force method implemented by invoke transmit method twice to make sure the data in the first transmission is already done. But in our case, we do not want to wait the second data exchange. We only care about the data is already flushed.