Incremental send/receive does not work with btrfs-progs-3.12-r1 and gentoo-sources-3.10.17, both in stable branch, as described here:
As stated in the messages above, btrfs-progs-3.12 should not be used with kernels <3.12
So upgrade your kernel or downgrade your tools.
I don't think it makes much sense to drop the stable keywords now, but I guess we could try to keep them in sync with gentoo-sources in future releases.
sys-fs/btrfs-progs-3.12-r1 not in portage anymore and over 2,5 years in btrfs development is a lot. Marking as OBSOLETE.