MINOR: stream: split stats_dump_full_strm_to_buffer() in two

MINOR: stream: split stats_dump_full_strm_to_buffer() in two

The function only works with the CLI's appctx and does most of the
convenient work of dumping a stream into a buffer (well, the trash
buffer for now). Let's split it in two so that most of the work is
done in a generic function and that the CLI-specific function relies
on that one.

The diff looks huge due to the changed indent caused by the extraction
of the switch/case statement, but when looked at using diff -b it's
small.