NickServ: DB Service for authentication and nick management

This initial implementation is not feature complete,
but is perhaps the minimum needed to be useful.
It does the following:
    * Allows users to authenticate
    * Registers authenticated users' nicks
    * Protects registered nicks from impersonation^

^ Important caveat: While it will force a nick change after a timeout,
it does not prevent the user from using the impersonated name within that time limit.
txircd.sql: Add some test data for donors
NickServ: implement logout
nickserv: Add GHOST command