State caching

This change introduces a global, per-state cache that keeps account data
in the canon state. Thanks to @karalabe for lots of fixes.

core/state: short-circuit balance change if zero value

light: fix memory expansion bug (same as fix for core/state)

ethclient: bugfix retrieving logs

core/state, light: remove unused StateObject.initCode

core/state: Fix memory expansion bug by not copying clean objects

cmd, eth: drop the blockchain version from cli/eth configs

miner: set tx index logs

rpc: format filter ID according to spec for quantities

core/types, ethclient: fix broken subscriptions

ethereum, ethclient: add SyncProgress API endpoint

cmd/evm, cmd/geth, cmd/utils: move version handling to cmd/utils

miner: Prevent attempts to close nil quit channel in agent (fixes #2948)

Transaction pool optimizations

contracts/ens: Replace setOwner with setSubnodeOwner per EIP137

swarm: plan bee for content storage and distribution on web3

Small docker run example for main README

This change imports the Swarm protocol codebase. Compared to the 'swarm'
branch, a few mostly cosmetic changes had to be made:

* The various redundant log message prefixes are gone.
* All files now have LGPLv3 license headers.
* Minor code changes were needed to please go vet and make the tests
  pass on Windows.
* Further changes were required to adapt to the go-ethereum develop
  branch and its new Go APIs.

Some code has not (yet) been brought over:

* swarm/cmd/bzzhash: will reappear as cmd/bzzhash later
* swarm/cmd/bzzup.sh: will be reimplemented in cmd/bzzup
* swarm/cmd/makegenesis: will reappear somehow
* swarm/examples/album: will move to a separate repository
* swarm/examples/filemanager: ditto
* swarm/examples/files: will not be merged
* swarm/test/*: will not be merged
* swarm/services/swear: will reappear as contracts/swear when needed

contracts/release: move package release to contracts/

contracts/ens: regenerate binding with solc v0.3.6