cls_rgw, rgw-admin: move bi_get() entry encoding to cls

Create a new structure to hold entry info, so that we can later creata a
bi_list() operation.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
cls_rgw, rgw-admin: create bi list operation

Can now list bucket index entries that correspond to a single key. This
is useful when debugging versioned objects issues.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>