rgw: clean up some code

rgw: clean up some code

In rgw_admin.cc a variable `f` is used frequently. However due to the
sheer size of the file and variable scopes involved, finding the
declaration of `f`, which is a common character, can be
challenging. So it has been renamed `stream_flusher`, which better
indicates its purpose and makes it easier to find the corresponding
declaration.

Additionally some unneeded variables in RGWBucket::check_object_index
were removed. rgw::sal::Bucket::ListParams initializes parameters as
one would hope/expect.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>