Allow multiple scheduled indexes to be created with a single planner run

When there are multiple indexes scheduled for background creation, allow multiple such indexes to be created using a single planner run.

This can be done as long as the optimal index placement is being guaranteed.

Note: some of the indexes may have input "nodes" specified, in which case, planner should only consider those indexer nodes - for creating those indexes. So in that case, single planner run may not be enough.