planCacheClear() currently only operate on system.buckets collections for legacy timeseries. 

We should:

• translate the namespace on the router side of the command, similarly to what we do for the listIndexes command (to support translation for tracked collections)
• translate the namespace on the shard side of the command, by substituting acquireCollection with acquireCollectionWithBucketsLookup (to support translation for untracked collections)