Fix #436 object_database created outside of witness data directory

[windycrypto]

PR for #436.

[windycrypto]

@pmconrad thanks for you reviews in #687:

IMO you shouldn't introduce an "_opened" flag without

1. specifying what it means that the database is "open", and
2. consistently checking the flag wherever it makes sense.
   For example, what happens if you create objects in a database that is not "open", and then call
   open(). Doesn't make sense, should be forbidden.

It seems that database::wipe is only ever called from application::startup. before database::open is called. A minimalist fix would be to simply remove the call to database::close() in database::wipe(). I haven't checked if close has any other relevant side effects though.

I have removed unrelated changes of .gitignore and added more comments for _opened.

IMO database::wipe() is to delete the index&data files, so it's properly database::close() it before deleting theses files. No guarantee database::wipe() only be called from application::startup() in the future.

[abitmore]

Looks fine for me.

[pmconrad]

Thanks!
The change has the desired effect.
The definition of "open" is not quite what I'd hoped for :-), but it's consistent and it's a step in the right direction.

[pmconrad]

...and database::close() has not been called, or failed during execution.

[windycrypto]

Thank you~ I have supplemented the comments.