- `IMAPConnection::openBox` now returns a Promise that resolves to an
IMAPBox, and IMAPBox contains all of the `fetch` operations. This makes
the dependency between fetch operations and the currently open mailbox
explicit rather than implicit and by forcing the operations to be called on
a box instance and hopefully prevent errors. It will also throw an error
if the constraint is no longer satisfied.
- `fetch` operations now return an observable stream of messages (or Promise for single value),
while preserving the same logic of the original implementation. You can use `.toPromise()` on
the observable to get a Promise that resolves when the observable stream has
completely drained.
- Fixes error handling in a few places and renames some variables
