Tunable replication frequency

Currently, replication happens when a write ahead log file is closed. The only parameter to toggle when this event occurs is write ahead log size, and is only applicable to the tablet servers themselves.

By default this means that when replication happens isn't tied to the table it is configured on, but also exogenous factors such as total write load and failures. If a system receives ~100MB/day/TServer, and the WAL size is its default 1GB, it will take 10 days for any replication event to occur. Another possibility is that an unreplicated table is receiving many writes, which will cause more frequent replication events, but proportionally the work will involve less data for the table being replicated.

I don't have a specific implementation in mind, but I'd like to see a solution that involves isolating the work down to specific table events such as time-since-last-replication and data-added-since-last-replication.

elserj has had some ideas about doing things incrementally within WAL files (ie, replicating between two sync points) that can also help with this.