Cassandra: Sequence generator tweak.

This tweaks the retry policy used for updating a Cassandra sequence in order to
perform better under heavy load.

This changes the retries to be done in the following msec intervals:
100 200 400 800 1600 3000 3000 3000 3000 3000
instead of the current: 
10 20 40 80 160 320 640 1000 1000 1000

This has been tested with 50 concurrent threads performing 20 sequence updates with 500 msec sleeps in between. The new setting turned out to perform much better under that load than the current. 

Note: When this is merged, we need to release a new toolbox version and start using it in the Purchase system in order to prevent incidents related high volume customers. 

Ping @adamvoncorswant