rgw/http: use a dedicated mutex for reqs_change_state

rgw/http: use a dedicated mutex for reqs_change_state

set_request_state() pushed into reqs_change_state without holding
any lock. Concurrent callers and manage_pending_requests raced on
the list and corrupted node links, crashing in std::list::_M_hook.

Use a dedicated mutex; reusing reqs_lock would invert the
completion path's reqs_lock -> req_data->lock order against the
set_request_state callers, which already hold req_data->lock.

Signed-off-by: Matthew N. Heler <matthew.heler@hotmail.com>