* swarm/storage/mock: implement listings methods for mem and rpc stores
* swarm/storage/mock/rpc: add comments and newTestStore helper function
* swarm/storage/mock/mem: add missing comments
* swarm/storage/mock: add comments to new types and constants
* swarm/storage/mock/db: implement listings for mock/db global store
* swarm/storage/mock/test: add comments for MockStoreListings
* swarm/storage/mock/explorer: initial implementation
* cmd/swarm/global-store: add chunk explorer
* cmd/swarm/global-store: add chunk explorer tests
* swarm/storage/mock/explorer: add tests
* swarm/storage/mock/explorer: add swagger api definition
* swarm/storage/mock/explorer: not-zero test values for invalid addr and key
* swarm/storage/mock/explorer: test wildcard cors origin
* swarm/storage/mock/db: renames based on Fabio's suggestions
* swarm/storage/mock/explorer: add more comments to testHandler function
* cmd/swarm/global-store: terminate subprocess with Kill in tests
