This patch is proposed to reduce the number of RPC for the large PUTs
The number and data size of write thread(SingleServerRequestRunnable) is a result of three main factors：
1) The flush size taken by BufferedMutatorImpl#backgroundFlushCommits
2) The limit of task number
A lot of requests created with less MUTATIONs is a result of two reason:
1) many regions of target table are in different server.
2) flush size in step one is summed by “all” server rather than “individual” server
This patch removes the limit of flush size in step one and add maximum size to submit for each server in the AsyncProcess