Portal master accumulator was removed from the network specs as a
content type shared on the network, as since the merge this is
a finite accumulator (pre-merge only).
So in this PR the accumulator gets removed as network type and
gets instead baked into the library. Building it is done by
seperate tooling (eth_data_exporter).
Because of this a lot of extra code can be removed that was
located in history_network, content_db, portal_protocol, etc.
Also removed to option to build the accumulator at start-up
of fluffy as this takes several minutes making it not viable.
It can still be loaded from a provided file however.
The ssz accumulator file is for now stored in the recently
created portal-spec-tests repository.
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