RenameDirectory state machine was not handling target not found correctly.
This would have caused asserts (which result in crashes in production builds)
There was also a bug in the rollback logic which would have caused a lingering
lock on the source link. While breaking assumptions this was a benign bug as
any operation on that directory would try and succeed acquiring this lock again.
It would succeed as lock requests are idempotent.
Things not done because probably disruptive:
* kmod filesystem string
* sysctl/debugfs/trace
* metrics names
* xmon instance names
Some of these might be renamed too, but starting with a relatively
safe set.
