[2109] ZoneFinder for the improved memory datasource

[2109] ZoneFinder for the improved memory datasource

Moved out the ZoneFinder-specific classes and code from memory_datasrc.[h|cc] into memory/zonefinder.[h|cc]

The new code is mostly similar to the original, but using the new data structures instead of the old. It is also no longer templated (not necessary anymore) and no longer uses the pimpl (also no longer relevant), so it should be somewhat easier to read and modify.