rhythm: fail safe when no partitions are assigned to a blockbuilder instance

[javiermolinar]

What this PR does:
Handle gracefully when a block-builder is initialized and it has no partitions assigned.

It also relies on the existing ring partition Ids to connect on instead of just looking on the number of active partitions

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[mapno]

getAssignedActivePartitions also needs to be fixed. That function assumes that if there are 5 instances in the partition ring, partitions 0, 1, 2, 3 and 4 are the ones active-which is not necessarily true, it can be any other existing partition in Kafka.