draft hard upper bound on data volume per flush to Cassandra

Cassandra is configured with a limit on the maximum size of a
Thrift operation, which bounds the amount of data that can be
flushed as one batch.

(Thanks to Ambuj Singh for the report!)
hard upper bound on data per flush to Cassandra, continued

(Thanks to Ambuj Singh for his input!)
Merge pull request #42 from wlam/cassandra-write-limit

Honor Cassandra single-batch write-size limit