Reuse the rpc send thread in datastream sender.
Reuse the rpc send thread in datastream sender.

We're currently making one per row batch which doesn't seem good. If anything,
it makes attaching gdb impossible.

Instead, I've switched to using a 1-thread thread pool. The thread pool handles
keeping that thread around and having a work queue. I've preserved the current
behavior that only 1 rpc is in-flight at a time.

Change-Id: I868565727ab9220c6b2e9ef95551e75ec1cd8c3c
Reviewed-on: http://gerrit.ent.cloudera.com:8080/428
Tested-by: jenkins
Reviewed-by: Marcel Kornacker <44592b4eea36663c86b994bb0ea99d15309c1c7d@cloudera.com>

michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala