cassandra: Improve snapshot_count_branches performance

It has been observed calling that method can lead to cassandra
query timeout when a snapshot contains a large amount of branches.

The performance bottleneck seems to be the use of a custom cassandra
aggregate function computing the number of branches per target type.

Removing the use of that function and performing the aggregation
client side significantly improves query performance and makes
the timeout issue goes away.

Fixes #4719.