DiskManager keeps temporary files around until the manager itself is dropped

[alamb]

Describe the bug

The DiskManager passes out Strings rather than TempFiles. The tempfiles are eventually cleaned up, but only when the DiskManager is droped , rather than the actual temp file use us is complete.

This both leaves temporary files around longer than necessary, and would cause trouble with a long lived DiskManager across plans.

To Reproduce
Make a shared DiskManager and run queries that spill to disk. The files will not be cleaned up until the DiskManager is dropped

Expected behavior
When the temp file is no longer in use, it should be dropped

Additional context
Noticed while working on #1668