Previously data returned from `memcache get` would be truncated at the
first NULL byte.

By using raw_write_function here to stream the returned memcache
value, we allow mod_memcache to be used for audio and other arbitrary
binary data.

Dave has a format module planned that relies on this.

Thanks-to: Dave Olszewski <dolszewski@marchex.com>

FS-7114 #resolve