augmented assignment operators should lock the left hand side

[mahrud]

Currently H.cache.blah ??= foo works as follows:

1. lock H.cache
2. attempt to read H.cache.blah
3. unlock H.cache
4. if successful, return value read in step 2
5. if it failed:
6. evaluate foo
7. lock H.cache
8. write H.cache.blah = foo
9. unlock H.cache

Problem: if H.cache is updated between steps 3 and 7, then the value is corrupted. This is also the case for any other augmented assignment operator, e.g. +=. Probably the solution is to lock the left hand side (which isn't always a hash table), but given the code it's unclear how to do this.

I mentioned this here, but it's becoming very critical, so wanted to make it its own issue.