rgw/cloud-transition: Fix issues with MCG endpoint

rgw/cloud-transition: Fix issues with MCG endpoint

Few s3 endpoints (like Noobaa MCG or AWS) may return BucketAlreadyOwned
or BucketAlreadyExists error if the target path (configured for
cloud transition) already exists. Do not fail transition in those cases.

Also fixed below issues -
* with racing lc threads, when one lc thread is trying to check and create the
target bucket, others may go ahead with the object transition assuming target path
exists.
* initialize few fields to avoid setting wrong content_len during transition

Signed-off-by: Soumya Koduri <skoduri@redhat.com>