rgw: add buffering filter to compression for fetch_remote_obj

rgw: add buffering filter to compression for fetch_remote_obj

fetch_remote_obj() only gets 16k blocks from libcurl, which leads to a
much worse compression ratio than the 4m blocks in normal PUT requests

Fixes: http://tracker.ceph.com/issues/23547
Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit 13160222fd6894a2851e61fd909d7332747ecdca)

 Conflicts:
	src/rgw/rgw_rados.cc (template functions for intarith macros
		were added in c06b97b3d7e36b5b3be38f1ffa121611bea61a52,
		and RGWGetDataCB was changed in
		b2143cded0e971361cdb089db19a6f69ce5b74dd)