Merge lp://staging/~cjwatson/storm/more-weakrefs into lp://staging/storm

Use weak references from Variable and MutableValueVariable to EventSystem, to avoid GC cycles.

Launchpad's test suite has had occasional problems for years with PostgresConnection leaks (Zope's test runner is careful about checking for GC garbage), but a combination of randomised test order and the fact that it often took hours to reach the failure on development systems even when we did manage to reproduce it meant that we had to work around it rather than fixing it properly; we've found that the problems are generally around dealing with MutableValueVariable instances. A couple of days ago, by chance, I ran across a much smaller subset of the Launchpad test suite that reproduced this bug, and so I was able to figure out a proper fix in Storm.

The problem was that, when change tracking was enabled on a MutableValueVariable, the variable became part of a reference cycle with the store via the event system. Using weak references for the event system appears to fix this, and I've been able to do a full successful Launchpad test run with this patch and with some of our workarounds reverted.