* swarm/network: integrity on incoming known chunks
* swarm/network: fix integrity check for incoming chunks
* swarm/storage: imrpoved integrity checking on chunks
* dbstore panics on corrupt chunk entry an prompts user to run cleandb
* memstore adds logging for garbage collection
* dbstore refactor item delete. correct partial deletes in Get
* cmd/swarm: added cleandb subcommand

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