Change LRUCache to be tree-based so we can delete subtrees.

synapse/push/push_rule_evaluator.py

synapse/util/caches/descriptors.py

synapse/util/caches/dictionary_cache.py

synapse/util/caches/lrucache.py

synapse/util/caches/treecache.py

tests/storage/test__base.py

tests/util/test_lrucache.py