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MessageCache: derive the hash from the cache contents

Description

MessageCache: derive the hash from the cache contents

Follow-up for I020617d, where I got this wrong. It is probable that multiple
application servers will try to build a local cache at the same time, in which
case they will cause the cache key to thrash. It is necessary that two
application servers building a local cache from the database generate the same
hash for a given set of messages.

Change-Id: Ieeefc2094a83be9401c466bec859c1588ddfbcdf