Prevent opening datastore as read-write when it is already opened in read-only mode

[liss-h]

Currently, metall prevents to open the same datastore as read-write twice at the same time.
However, it allows to open a datastore as read-write even if it is already opened in read-only mode.

This causes issues/corruption when the read-writer actually writes. Metall should prevent this.

The desired behaviour would be, allow either:

• exactly one read-writer, no one else
• many readers, but no read-writer

Small snippet to reproduce:

{ // create datastore
    metall::manager mgr{metall::create_only, path};
}

metall::manager mgr_ro{metall::open_read_only, path};
metall::manager mgr_rw{metall::open_only, path}; // this should fail

[KIwabuchi]

That's a good point. I'll work on that.