We already have 3 commands in zodbtools suite (zodbanalyze, zodbdump &
zodbcmp) and this is going to grow. And it was already noted some time
ago with TODO (in 66946b8d) that we need only one command driver to
invoke everything.

So do it: introduce `zodb` command which can invoke other subcommands
and show general help or help for subcommand or a topic.

The structure is modelled after `git` and `go` commands. Help topics
are for now empty but we'll add one help topic in the next patch.