client: avoid an extra construction of ColumnSchema
client: avoid an extra construction of ColumnSchema

KuduSchema::Column() was making an unnecessary copy of a ColumnSchema
object, when it instead could just use a local reference. This showed up
in my TSBS benchmark as an easily-addressed waste of CPU cycles.

Change-Id: I695d760837fb529a549e83b5385a1df79f358697
Reviewed-on: http://gerrit.cloudera.org:8080/15501
Reviewed-by: Andrew Wong <b68e4fdc6430321a6b47400732ff97d7ae91234e@cloudera.com>
Reviewed-by: Bankim Bhavsar <3aac13853bde1b179487acd91435d1ded94913cb@cloudera.com>
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>
Reviewed-by: Alexey Serbin <864f6b82bd94e8dda64141d4274519ee66e0102e@cloudera.com>
Tested-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>

helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu