Merge pull request #10767 from rzarzynski/wip-rgw-frontend-rework

rgw: frontend subsystem rework

all requested changes have been made by the author
tested by-hand w/sigv2 and sigv4, and already passed teuthology rgw suite

diff --cc src/rgw/rgw_op.cc
Simple merge

diff --cc src/rgw/rgw_rados.cc
Simple merge

diff --cc src/rgw/rgw_rados.h
Simple merge

diff --cc src/rgw/rgw_rest.cc
Simple merge

diff --cc src/rgw/rgw_rest_s3.cc
Simple merge