if “20,000 concurrent writes” means inserts then I would go for CouchDB and use “_changes” api for triggers. But with 20.000 writes you would need a stable sharding aswell. Then you would better take a look at bigcouch
And if “20.000” concurrent writes consist “mostly” updates I would go for MongoDB for sure, since Its “update in place” is pretty awesome. But then you should handle triggers manually, but using another collection to update in place a general document can be a handy solution. Again be careful about sharding.
Finally I think you cannot select a database with just concurrency, you need to plan the api (how you would retrieve data) then look at options in hand.