Stratum server support for GBT upstreams

This uses new, as-of-yet unmerged libblkmaker code:
https://gitorious.org/bitcoin/libblkmaker/commits/9f5e949f7e1e0bb49bfe8eab1d99107b3114b4c7

It also moves 2D work code out of the stratum proxy, so other drivers can potentially be added using it also.