deprecate find_references calls during AsdfFile.__init__ and open

[braingram]

Description

Deprecate the automatic calling of AsdfFile.find_references during AsdfFile.__init__ and asdf.open. A warning will only appear if a reference is found. References appear to be unused in all known downstream packages so I don't expect we will see any of these warnings. find_references walks and modifies (so performs a deep copy of) the tree so the eventual removal of this automatic calling should speed up most calls to AsdfFile.__init__ and asdf.open.

Checklist:

• pre-commit checks ran successfully
• tests ran successfully
• for a public change, a changelog entry was added
• for a public change, documentation was updated
• for any new features, unit tests were added

[eslavich]

Gain some time back, lose... nothing useful. If super-dictionaries catch on then we'll have a natural way to lazy load references too.