Conditional update based on UDF

Building on what is suggested by: CASSANDRA-8488

It would be great if conditional updates could accept a UDF in the IF clause.

Use Case:
We are doing real-time in-memory aggregations across many machines. We want to update a value (e.g. a dimensional count), but we only want the update applied if that time bucket has not already been incorporated by another machine. (protecting against partitioning, replays, etc.)

In this case, we want something like:
UPDATE value=v, slices+="1123213222-30" IF not_contains(slices, 1123213222-30)

Where "slices" is a column that contains all time slices, and 112313222-30 represents a slice of time (start time, plus interval).