[FTS] stats polling should use cached indexDefs/planPIndexes from metakv

ns-server polls cbft every second for stats, and cbft in turn calls metakv to retrieve the latest indexDefs and planPIndexes... which can be expensive.

for stats, consider caching those for a bit (or until there's a runObserveChildren event/change) in order to hit ns-server/metakv less often.