Most of our tools need only read access for working. However e.g.
FileStorage, when opened in read-write mode, automatically creates
database file and index.

This way if database is opened in read-write mode a simple typo in path,
e.g. to `zodb dump path` would lead to:

- new database at path will be created
- the dump will print nothing (empty database)
- exit status will be 0 (ok) and no error will be reported.

For this reason it is better tools declare access level they need so for
read-only access request we can catch it with an error from storage.

This, however, requires quite recent ZODB to work:

https://github.com/zopefoundation/ZODB/pull/153

P.S.

We don't want to force users to always specify read-only in URLs or
zconf files because:

- this is error prone
- URL or zconf can be though as of file
- when a program opens a file the program, not file, declares which type
  of access it wants.

That's why access mode declaration has to be internal.