Add JoinKeysIterator() to strings/join.h
Add JoinKeysIterator() to strings/join.h

This is a simple function that iterates over map keys.

Change-Id: I3b1376f12d151591d93988cee217f1ecb1bcd224
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4831
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>
Tested-by: jenkins

andrwng/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,helifu/kudu,andrwng/kudu,InspurUSA/kudu,andrwng/kudu,InspurUSA/kudu,helifu/kudu,helifu/kudu,cloudera/kudu,andrwng/kudu,EvilMcJerkface/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,helifu/kudu,cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,helifu/kudu,cloudera/kudu,helifu/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,cloudera/kudu,EvilMcJerkface/kudu,andrwng/kudu,andrwng/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,cloudera/kudu