rgw: more refactoring work

rgw: more refactoring work

notable changes are around user metadata. Create an api that uses
the service interface (that requires backend context) and use it
for higher level functions. Still a lot to do.

Following rebase, started modifying the internal meta apis to deal
with optional_yield param. Passing it from top level metadata handlers
and from ctls, all the way down to the services. However, since following
commit will introduce many more changes, this was only done for the
user apis.

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