Add a counter, as a potential cache key

[hannosch]

Plone patches in a counter (a BTrees.Length) object onto the catalog which gets updated every time any object is added/changed/removed from the catalog. See https://github.com/plone/Products.CMFPlone/blob/master/Products/CMFPlone/CatalogTool.py#L395

Adding a similar counter to the event index (triggered in un/index_object) would provide a powerful event specific cache key. Using such a key one can add query caches, which get properly invalidated on event changes alone, so one can do these persistently or share them via memcached. One could even add such a cache to the index itself, with some configurable number of items in a LRU cache.