Datafacades assume data layout is in a contiguous memory block

[danpat]

Currently, it is only possible to implement alternative datafacades using the all-in-one-block structure described by the ContiguousInternalMemoryDatafacade.

If someone would want to implement an alternative storage approach - e.g. keeping data in individual files and mmap()-ing them, then our current inheritance structure makes this impossible - the algorithm-specific classes assume they're based on the ContiguousInternalMemoryDatafacade rather than the BaseDatafacade.

We should refactor this so that alternative storage can be implemented without needing to re-write the algorithm-specific subclasses.

[TheMarex]

I talked with @oxidase about this yesterday a little bit an we concluded the best option here would be a compile time switch, since we don't need the flexibility of switching this at run-time.

There are two methods to do this:

1. Go full template. This is undesirable on multiple levels and chances are we would never use this functionality.
2. Compile-time switch using a type alias. For external embedders of OSRM this should work just as well.

[github-actions[bot]]

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