This adds a swarm ls command which lists files and directories stored in a
manifest. Rather than listing all files, it uses "directory prefixes" in case there are a
lot of files in a manifest but you just want to traverse it.
This also includes some refactoring to the tests and the introduction of a
swarm/api/client package to make things easier to test.
* accounts, cmd, eth, ethdb: port logs over to new system
* ethdb: drop concept of cache distribution between dbs
* eth: fix some log nitpicks to make them nicer
