nsIDBChangeListener
only used by nsinewsdatabase databases.

MailNews Protocols
these implement nsimsgprotocol, inherit from nsmsgprotocol, and implement their own protocol-specific interface (nsiimapprotocol, nsipop3protocol, nsinntpprotocol) in addition, in mailnews/db/msgdb/public, we have protocol-specific msg db classes (and in the case of news, a nsinewsdatabase interface), so that we can use polymorphism when msg db operations need to be specialized for different kinds of folders.