core: move transaction's sender to cosigners

Closes #1184

Ported changes from https://github.com/neo-project/neo/pull/1752

pkg/core/util_test.go

@@ -21,7 +21,7 @@ func TestGenesisBlockMainNet(t *testing.T) {
 	// have been changed. Consequently, hash of the genesis block has been changed.
 	// Update expected genesis block hash for better times.
 	// Old hash is "d42561e3d30e15be6400b6df2f328e02d2bf6354c41dce433bc57687c82144bf"
-	expect := "94e61af2441145cc251752707a58107850328a48bb095fd175ca2f8513ab5676"
+	expect := "ecaee33262f1bc7c7c28f2b25b54a5d61d50670871f45c0c6fe755a40cbde4a8"
 	assert.Equal(t, expect, block.Hash().StringLE())
 }