Use a std::map instead of a std::list of std::pair.

Use a std::map instead of a std::list of std::pair.

The only relevant change is that where previously, when we unpacked one
data set in p::d::Triangulation::notify_ready_to_unpack(), we just dropped
*one* data set but not necessarily the one that had been detached. Now we
actually drop the *correct* one by searching for the key and dropping
that element of the map.