Determine where entities are stored without knowing their types

It should be possible to find an entity solely based on its UUID, without needing to know the entity type. That's really the whole point of UUIDs. Users or callers shouldn't need to care what an entity's type is.

This would allow for things like:

• surfing to /entity/UUID via #2353611: Make it possible to link to an entity by UUID and/or #1726734: Replace serial entity IDs with UUIDs in URLs, or even globally?,
• calling EntityRepository::loadEntityByUuid() without needing the $entity_type_id parameter,
• etc.

A discussion on this topic was started at #1637370-55: Add UUID support to core entity types, with the idea of "adding a uuid lookup table to core in addition to the uuid column in the entity base table":

The global lookup table mapping UUID to entity (in addition to the entity specific column) would be analogous to the global url alias table. That becomes crucial if we change system urls to reference UUID instead of nid, uid, etc. If we do that, we might have to dereference URLs in a service like #1269742: Make path lookup code into a pluggable class.