move remote eth backend from ethdb to core package (#1567)

cmd/rpcdaemon/cli/config.go

cmd/state/commands/gas_limits.go

cmd/state/commands/state_growth.go

common/dbutils/composite_keys.go

common/dbutils/composite_keys_test.go

core/eth_backend.go

ethdb/kv_abstract_test.go

ethdb/kv_remote.go