#10 use last update time from source

[Bryan Tripp]

IChunk.getLastUpdateTime() was originally meant to
reflect the time at which the source system changed the
data. It was meant to be used in eviction rules that
treat recently changed data as more likely to be stale.

Along the way this was reinterpreted to mean the time
at which the cached record was last updated (this
ignores whether the data value changed with the update,
and also whether the data haven't changed for a long
time prior to being cached).

IChunkStore.put() should have last update as an
additional arg, and it should be derived from the most
recent IDataItem.getLastUpdate() in the chunk.

If we want to use the other interpretation, it should
be done by updating the cache time with each put (I
think this is done currently anyway).