Move code to load User data from cache out of loadFromId

Move code to load User data from cache out of loadFromId

This way, outside code can load data from cache only if present. E.g. when data
for a lot of users is needed, it'd be preferable to batch-load all that is not
yet in cache.

Since there's currently no way to check for data in cache, one would have to
load all User objects separately (which may result in lots of DB queries if
nothing is in cache) or batch-load all of them (which may result in few DB
queries, but ones that may not be needed if all data is already in cache)

Change-Id: Ia22d04744760c3b6f3264786434e218b8aa88d53