don't use capped collection -- too inflexible

Using a capped collection makes it troublesome to modify posts and impossible to delete them (we could void them, but they'd still contribute to the max number of posts).

To convert the existing chat_dbs to a non-capped collection:

use livechan_db
db.chat_dbs.copyTo("chat_dbs2")
db.chat_dbs.drop()
db.chat_dbs2.renameCollection("chat_dbs")