rgw: rename search variable to better indicate semantics

rgw: rename search variable to better indicate semantics

cls_cxx_map_get_vals takes a parameter of where in the sequence of
keys to start returning values. In code it's often referred to as the
"start_key", but in fact it only returns values who have keys *after*
it. To clarify the semantics, rename the variable to "start_after_key"
in the bucket listing functions in both rgw and cls.

Also, declare a type RGWRados::check_filter_t, so the type does not
have to be fully literalized multiple times.

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