Make the contracts cache initialization unified. The order of cache
iniitialization is not important and Nottary contract is added to the
bc.contracts.Contracts wrt P2PSigExtensions setting, thus no functional
changes, just refactoring for future applications.
Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>
There is a security issue found in github.com/btcsuite/btcd that we don't care
about (we're only using 256k1 implementation), but GitHub complains about
it. We could update to github.com/btcsuite/btcd/btcec/v2, but it's now just a
thin wrapper over github.com/decred/dcrd/dcrec/secp256k1/v4, so we better use
it directly.
