rgw: normal PUT object requests got 405 sometimes

Under load testing, normal PUT object requests got 405 sometimes. Bug #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(). But it's safer to make sure the hex table is initialized before first use. The easiest approach is wrapping the table as a class and defining as a static local variable in hex_to_num() as commit 5032ca2b4abf701a22f9a71a57d48c221a8d935e.