IMPALA-825: Synchronize Hive Metastore client connection creation

The creation of hive metastore clients is now synchronised to avoid the possibility of
race conditions accessing local Kerberos state. In experiments, this does not fully
resolve the issue but significantly reduces the chances it will occur.

Also adds in a new debug config option to optionally sleep for a specified number of
milliseconds between creating connections. The default is zero, but can be configured
by setting impala.catalog.metastore.cnxn.creation.delay.ms in the hive-site.xml.

Change-Id: I83e863760470bdc2d9b27c6669f35604111d69d7
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2661
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins