core: optimisations

Conflicts:
	core/chain_manager.go
	miner/worker.go

With blocks now being immutable, use big.Int values from
accessor functions instead of copying their results.

This pleases the RPC tests.

TestMadeupParentBlockChainAttack has been deleted because it was too
hard to port and the attack that it checks the prevention of is being
averted in a different way (through a protocol change).

Debug develop

eth/fetcher: don't double filter/fetch the same block