IMPALA-6395: Add a flag for data stream sender's buffer size
IMPALA-6395: Add a flag for data stream sender's buffer size

This change adds a flag to control the maximum size in bytes
a row batch in a data stream sender's channel can accumulate
before the row batch is sent over the wire. Increasing this
value will better amortize the cost of compression and RPC
per row batch. The default value is 16KB per channel.

Change-Id: I385f4b7a0671bb2d7872bee60d476c375680b5c2
Reviewed-on: http://gerrit.cloudera.org:8080/9026
Reviewed-by: Michael Ho <43544f6c6f19372b140301f92a3768afc300868a@cloudera.com>
Tested-by: Impala Public Jenkins

cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala