listCollections should not require a MODE_S database lock

Currently the listCollections command requires a MODE_S lock on the database it's run on. When any transaction is active on that database, it will generally have a MODE_IX lock on a collection. This will conflict with the MODE_S lock on the database, causing listCollections to block until the transaction is finished or aborted.

It should be sufficient to have the MODE_IS lock on the database, as long as each collection is locked as its details are gathered.