IMPALA-7565: Add startup flag to set thrift connection setup thread pool size
IMPALA-7565: Add startup flag to set thrift connection setup thread
pool size

This patch adds a hidden experimental startup flag called
'accepted_cnxn_setup_thread_pool_size' which enables setting the size
of the thread pool used to process the internal post-accept, pre-setup
connection queue in each thrift server set up to service
Impala internal and external connections. The default is set to 1
which ensures that this change does not affect current behavior.

Testing:
Tested manually by adding a sleep and making sure other threads are
used.
Ran exhaustive tests with a pool size set to 10 successfully.

Change-Id: I31344321a5f9e840a399ccb0f963c0759e2ab234
Reviewed-on: http://gerrit.cloudera.org:8080/11873
Reviewed-by: Impala Public Jenkins <039b6b20426ff68ebe492b3021890d8c8fd010c9@cloudera.com>
Tested-by: Impala Public Jenkins <039b6b20426ff68ebe492b3021890d8c8fd010c9@cloudera.com>

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