RS: BatchActor over-subscribes if flushInterval is used

[rkuhn]

Using the flushInterval will lead to less than batchSize elements being written in a batch, but when processing the ResultSet a full batchSize is requested from upstream, leading to unbounded growth of outstanding work (by violating the concurrentRequests limit).

[rkuhn]

The BatchActor will have to keep track of the size of each write batch and then only request that number of new elements when the database result comes back.