swarm/network: fix chunk integrity checks (#3665)

* 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