Support weak references

[avish]

We ran into an issue trying to create weak references to LRU objects: the current implementation of LRU does not support weak references. It would be straightforward to implement according to the guidelines from https://python.readthedocs.io/en/v2.7.2/extending/newtypes.html#weak-reference-support, and would make the LRU class more usable in advanced scenarios, such as monitoring transient caches without keeping a hard reference to them.