Trigger client reconciliation based on manager poll

Currently the reconciliation logic on the client is triggered when a new target assignment is received and resolved, or when new unresolved target assignments are discovered in metadata.

This could be improved by triggering the reconciliation logic on each poll iteration, to reconcile whatever is ready to be reconciled. This would require changes to support poll on the MembershipManager, and integrate it with the current polling logic in the background thread. Receiving a new target assignment from the broker, or resolving new topic names via a metadata update could only ensure that the #assignmentReadyToReconcile is properly updated (currently done), but wouldn't trigger the #reconcile() logic, leaving that to the #poll() operation.