INonceStore assumes that a lock around the calls to ContainsNonce and
StoreNonce will protect against nonce name collisions.  In a web farm
environment this is not the case, and a shared database backend for the
nonce table will generate occasional exceptions or even allow replay
attacks in certain race condition scenarios.

We need to alter the INonceStore methods to include a built-in atomic
check-existence-and-add semantic method to replace the Contains and Store
individual methods.