core, eth, miner, xeth: clean up tx/receipt db accessors

core/block_validator_test.go

core/blockchain.go

core/blockchain_test.go

core/database_util.go

core/database_util_test.go

core/genesis.go

core/transaction_util.go

eth/backend_test.go

eth/filters/filter_test.go

miner/worker.go

xeth/xeth.go