Enhance the BulkWriteProcessor (#63)

1. Remove DataCarrier.
2. Make the bulk process in sync mode. Once the bulk size is reached, flush(gRPC) process starts immediately.
3. Add a timer as additional processor when the data slightly overs the bulk size.
4. Expose the gRPC timeout for bulk processor creation. Fix previous misused flushInterval as timeout.
5. Correct concurrency implementation. This OAP concept means, the max accepted sources to flush data concurrently. But the threads are controlled by the OAP persistent timer thread pool.

Author: wu-sheng