* 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