Fix critical bug in delete_tree

[danielballan]

The delete_tree deletes nodes recursively. It was added specifically to support tiled serve directory --watch ..., which blows and rebuilds the database when files change (as an initial simple/safe implementation).

Crucially, it can only ever delete registered data, never user-uploaded data. Which is a good thing because, as implemented, it always deletes the entire tree---not from a prescribed node downward but always from the root. Bad bug!

This bug was masking other bugs, also fixed here. Needs tests, but here's a demo of the intended behavior working as planned on this PR branch:

In [1]: from tiled.client import from_uri, show_logs, tree

In [2]: c = from_uri('http://localhost:8000', api_key='secret')

In [3]: c.create_container(key='A')
   ...: c.create_container(key='B')
Out[3]: <Container {}>

In [4]: c['A'].create_container('a1')
Out[4]: <Container {}>

In [5]: tree(c)
├── A
│   └── a1
└── B

In [6]: c['A'].delete_tree()

In [7]: tree(c)
├── A
└── B

In [8]: c.delete('A')

In [9]: tree(c)
└── B

Checklist

• Add a Changelog entry
• Add the ticket number which this PR closes to the comment section