[plasma] Examine existing resources in use to support/deny new index creations

Currently GSI does not examine the number of existing indexes and the system resources consumed by them, when a new index is created.

So it is possible that a system could end up having more indexes than its system resources will accommodate. For example, eventually OOM killer can kill the indexer process.

Instead it would be better, at index creation time, to look at the minimum resources needed for existing indexes to conclude if a new index create command can be supported or not.

This will allow customers to better size their system at creation time rather than wait for the system to halt with out of memory errors before expanding.

Suggestions:

1. We need a way to have new index creations to be able to aggressively reclaim resources from existing indexes when in pressure. (Currently it is a lazy approach).
2. Indexer should throw up WARNING messages (similar to disk full warning) when the system resources like memory are approaching minimum limits.