[rgw] put-obj-sometimes-got-405

Under load testing, normal PUT object requests got 405 sometimes. Fix #6672 (http://tracker.ceph.com/issues/6672) has been filed to track this issue. The root cause is that there is a defect in hex_to_num() to decode url in multi-threading context.

A quick fix is to set initialized to true after populating table in hex_to_num().