Hide multi-master cache coherency issues on preferences

User preferences are held in memory in the accounts cache, but can be
stale in a multi-master configuration due to writes being handled on a
different server process.  Paper over the lack of cache coherency by
reading the preferences from the database anytime they are requested
through the RPC or REST APIs.

Change-Id: I2a14e1f50e908a1d5ea5628e5a05e392be2e0d42
3 files changed