Batch writes by tablet server

Need the ability to batch writes from multiple thread writing to the same tserver. The batch writer can do this, however it has the following two drawbacks.
1. When flush is called on a batch writer it blocks all new calls to add mutations until the flush is finished. This was done to prevent the batch writer from using more than its configured memory. However this behavior could be changed w/o relaxing the memory limits.
2. When flush is called, a thread must wait for all mutations added by other threads to complete. It would be nice if a thread continued as soon as its mutations were written.

There is code in Fluo called the shared batch writer that gets around # 1, but it can not get around # 2. Seems like getting around # 2 will require changing Accumulo.