fastd: reorganize patches for easier backporting

Gluon v2020.2.x uses fastd v19, so we keep that in a separate patch. The
fastd memory leak fix from v18 is not removed in this patch anymore, as
the fix is needed for v19 as well.

The v20 and v21 patches are squashed into one, as they aren't backports
anymore after the rebase onto current openwrt-19.07.