rgw: move some bucket index delimiter filtering logic to cls

rgw: move some bucket index delimiter filtering logic to cls

Currently the cls code that returns bucket index entries does no
filtering when a delimiter is specified. This means that the cls code
will return many entries that the rgw code does not need. This means
unnecessary bucket index entries will be encoded, decoded, and consume
network bandwidth.

A change is made to send a delimiter, if it's specified, to the cls
code, so the filtering can be done there.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>