rgw: tie opstate into intra-region copy operations

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: buffer atomic put handler

Since we tied the atomic put handler to libcurl output
data, which uses much smaller chunks, we need to buffer
data, otherwise we'd end up with a huge amount of small
writes.

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