This fixes use-after-free bug in SimpleDatabase::Close(), accessing
the `root` object which was already freed by the `catch` block in
Open().

By having the Database on the stack first, we can avoid calling
Close() on the failed-to-open Database from Instance's destructor.

Closes #482

Make it less generic, to avoid clashes.

Closes GH-479.

Closes #477

Destruct automatically, even if leaving the scope due to exception
being thrown.

The list of partitions is cleared automatically.

This replaces the old `dead` flag which was unreliable; it was `false`
if the EventThread was not yet started, which could cause deadlocks in
BlockingCall().

They will be stopped by ~EventThread() when the `Instance` is deleted.