[GSI] Stale=ok scans are now allowed during indexer warmup

Linked bug: MB-32101
Any scan was not allowed earlier while indexer was in bootstrap mode (ie until all indexes were not warmed up). We have done an improvement now to allow scans during indexer warmup:

• Allow scans while indexer is still in bootstrap as and when indexes warm up from storage.
• Only scans for warmed up indexes are allowed.
• Only stale=ok OR scans for which a consistent snapshot is available are allowed.
• Error returned in other cases so that replicas can be tried.