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.

- Nexedi adheres to GPLv3+ with wide exception for Free/Open-source
  software. Use this for our original work.

- Zodbanalyze was initially developed by Zope corp & co. See

  d86d04dc	(initial copy of ZODB3-3.9.7's ZODB/scripts/analyze.py.)
  ab17cf2d	(Hook in Nexedi's zodbanalyze)

  So follow original Zope's licensing terms for zodbanalyze.

The package was named zodbtools from start (see fd6ad1b9 "Start of
zodbtools.git"). However in 66a03ae5 I've made a thinko naming some
parts as zodbtool.

Fix the confusion and name things uniformly as zodbtools.

/reviewed-by TrustMe

zodbanalyze was added in 1e506a81 and ab17cf2d.

/reviewed-on !1
/see-also slapos!116

This is enhanced version of ZODB original scripts/analyze.py .
Nexedi version was originally developed as part of ERP5 here:

https://lab.nexedi.com/nexedi/erp5/commits/master/erp5/util/zodbanalyze

It was agreed to move zodbanalyze to zodbutils

/see nexedi/slapos!116

* it is faster.
* it does not require Products.

(where `python setup.py sdist` puts result tarballs)

WARNING output format is not stable yet.

This is a tool to compare two ZODB databases in between tidmin..tidmax
transaction range with default range being -∞..+∞ - (whole database).

For comparision both databases are scanned at storage layer and every
transaction content is compared bit-to-bit between the two. The program stops
either at first difference found, or when whole requested transaction range is
scanned with no difference detected.

Database storages are specified in files with ZConfig-based storage definition, e.g.

    %import neo.client
    <NEOStorage>
        master_nodes    ...
        name            ...
    </NEOStorage>

Please see nexedi/neoppod!4 for
one of possible contexts.

The tool is generic though and is not NEO-specific. It should be able to
even check two different storages like ZEO & NEO, or FileStorage and NEO
etc and thus can be handy.

TODO tests

Project to be a place for various ZODB utilitieis. Jim Fulton suggested
to have it separate from ZODB/scripts/ [1] and now we have it here.

[1] https://github.com/zopefoundation/ZODB/pull/128#issuecomment-260970932