Add camel_imapx_stream_nstring_bytes().

Add camel_imapx_stream_nstring_bytes().

Replaces camel_imapx_stream_nstring_stream().

Produces a GBytes instead of a CamelStreamMem.  GBytes works better
than GOutputStream would because it can be reused without seeking to
the beginning.

This basically converts message fetching to use GIO streams directly.