* accounts, eth, clique: implement external backend + move sighash calc to backend

* signer: implement account_Version on external API

* accounts/external: enable ipc, add copyright

* accounts, internal, signer: formatting

* node: go fmt

* flags: disallow --dev in combo with --externalsigner

* accounts: remove clique-specific signing method, replace with more generic

* accounts, consensus: formatting + fix error in tests

* signer/core: remove (test-) import cycle

* clique: remove unused import

* accounts: remove CliqueHash and avoid dependency on package crypto

* consensus/clique: unduplicate header encoding