eth: add debug_storageRangeAt

The 'step' method is split into two parts, 'peek' and 'push'. peek
returns the next state but doesn't make it current.

The end of iteration was previously tracked by setting 'trie' to nil.
End of iteration is now tracked using the 'iteratorEnd' error, which is
slightly cleaner and requires less code.

Make it so each iterator has exactly one public constructor:

- NodeIterators can be created through a method.
- Iterators can be created through NewIterator on any NodeIterator.

'encode' and 'decode' are meaningless because the code deals with three
encodings. Document the encodings and give a name to each one.

cmd/bootnode: stop after generating/writing nodekey

* internal/ethapi: return empty arrays instead of null

* internal/ethapi: minor comments to avoid future regressions

core, light: delete SplitStatTy, ChainSplitEvent (unused)

cmd/faucet: further user validations and bot protection

cmd, node: add --nousb and node.Config.NoUSB to disable hw wallets

core: make genesis incompatibility error more explicit

The key was constructed from nibbles, which isn't possible for all
nodes. Remove the only use of Key in LightTrie by always retrying with
the original key that was looked up.

rpc: improve BlockNumber unmarshal parsing

cmd/geth: update flag groups in the geth command usage

common/compiler: add metadata output for solc > 0.4.6

build: bump launchpad builds to Go 1.8.1

Fixes #3608

consensus/ethash: simplify concurrency in VerifyHeaders

Currently http cors and websocket origins are a comma separated string in the
config object. These are replaced with string arrays that are more expressive in
case of a config file.

This change removes a convoluted use of sync/atomic from VerifyHeaders.
It also fixes the annoying error about future blocks.