- Backported 0.7's identity map implementation, which

- Backported 0.7's identity map implementation, which
does not use a mutex around removal.  This as some users
were still getting deadlocks despite the adjustments
in 0.6.7; the 0.7 approach that doesn't use a mutex
does not appear to produce "dictionary changed size"
issues, the original rationale for the mutex.
[ticket:2148]