Keeps the RoleGraph up to date in mongod, and converts MongoS to simply
ask the config server for required information about users and roles.
Performs somewhat aggressive cache invalidation of the users cache in mongod.
Still no cache invalidation in mongos.
Also requires changing the privileges of the built-in roles. This patch takes the opportunity to remove the 2.2-style read-only roles in favor of the 2.4-style "read" and "readAnyDatabase" roles, and renames the 2.2-style read-write roles "dbOwner" and "root". The "root" name, at least, is subject to change prior to the next unstable release.
Test harnesses are updated as needed to use the correct builtin roles.
