Optimisations and fixed a couple of DDOS issues in the miner

cmd/mist/assets/examples/bomb.html

cmd/mist/gui.go

core/block_processor.go

core/chain_manager.go

core/error.go

core/events.go

core/state_transition.go

core/transaction_pool.go

ethutil/common.go

miner/worker.go

pow/ezp/pow.go

rpc/packages.go

state/state_object.go

state/statedb.go

xeth/xeth.go