rgw: adding request context structure

rgw: adding request context structure

this structure should be created at the frontend and trickle all the way
to the RADOS layer. holding: dout prefix, optional yield and trace.
in this commit, so far it was only added to the "complete()" sal interface,
and to the "write_meta()" rados interface.

in the future, it should be added to more sal interfaces, replacing the
current way where dpp and optional yield are passed as sepearte
arguments to all functions.
in addition, if more information would be needed, it should be possible
to add that information to the request context struct without changing
many function prototypes

basic test instructions:
https://gist.github.com/yuvalif/1c7f1e80126bed5fa79345efb27fe1b1

Signed-off-by: Yuval Lifshitz <ylifshit@redhat.com>