And simplify atomic service value stored by RPC server. Oracle service can
either be an untyped nil or be the proper non-nil *oracle.Oracle.
Otherwise `submitoracleresponse` RPC handler doesn't work properly.
Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>
And include some node-specific configurations there with backwards
compatibility. Note that in the future we'll remove Ledger's
fields from the ProtocolConfiguration and it'll be possible to access them in
Blockchain directly (not via .Ledger).
The other option tried was using two configuration types separately, but that
incurs more changes to the codebase, single structure that behaves almost like
the old one is better for backwards compatibility.
Fixes#2676.
