librgw: header and namespace reorganization

librgw: header and namespace reorganization

Move RGWLibProcess and RGWLibFrontend into new rgw_lib_frontend.h
header, which can include rgw_lib.h and rgw_file.h.

This allows the already-defined RGWLibProcess to call methods on
RGWLibFS objects (e.g., to perform GC, which follows in a separate
commit).

In addition, move all of the library and file interface into
namespace rgw.

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>